The classic signs and symptoms of measles include four-day fevers (the 4 D's) and the three C's—cough, coryza (head cold, fever, sneezing), and conjunctivitis (red eyes)—along with fever and rashes. Fever is common and typically lasts for about one week; the fever seen with measles is often as high as . Koplik's spots seen inside the mouth are pathognomonic (diagnostic) for measles, but are temporary and therefore rarely seen. Recognizing these spots before a person reaches their maximum infectiousness can help physicians reduce the spread of the disease.

The characteristic measles rash is classically described as a generalized red maculopapular rash that begins several days after the fever starts. It starts on the back of the ears and, after a few hours, spreads to the head and neck before spreading to cover most of the body, often causing itching. The measles rash appears two to four days after the initial symptoms and lasts for up to eight days. The rash is said to "stain", changing color from red to dark brown, before disappearing. Overall, the disease from infection with the measles virus usually resolves after about three weeks.

Complications with measles are relatively common, ranging from mild complications such as diarrhea to serious complications such as pneumonia (either direct viral pneumonia or secondary bacterial pneumonia), bronchitis (either direct viral bronchitis or secondary bacterial bronchitis), otitis media, acute brain inflammation (and very rarely SSPE—subacute sclerosing panencephalitis), and corneal ulceration (leading to corneal scarring). Complications are usually more severe in adults who catch the virus. The death rate in the 1920s was around 30% for measles pneumonia. People that are at high risk for complications are: Infants and children aged 20 years, pregnant women, and people with compromised immune systems, such as from leukemia and HIV infection.

Between 1987 and 2000, the case fatality rate across the United States was three measles-attributable deaths per 1000 cases, or 0.3%. In underdeveloped nations with high rates of malnutrition and poor healthcare, fatality rates have been as high as 28%. In immunocompromised persons (e.g., people with AIDS) the fatality rate is approximately 30%. Risk factors for severe measles and its complications include malnutrition, underlying immunodeficiency, pregnancy, and vitamin A deficiency. Even in previously healthy children, measles can cause serious illness requiring hospitalization. One out of every 1,000 measles cases will develop acute encephalitis, which often results in permanent brain damage. One or two out of every 1,000 children who become infected with measles will die from respiratory and neurologic complications.

Rubella has symptoms that are similar to those of flu. However, the primary symptom of rubella virus infection is the appearance of a rash (exanthem) on the face which spreads to the trunk and limbs and usually fades after three days (that is why it is often referred to as three-day measles). The facial rash usually clears as it spreads to other parts of the body. Other symptoms include low grade fever, swollen glands (sub-occipital and posterior cervical lymphadenopathy), joint pains, headache, and conjunctivitis.

The swollen glands or lymph nodes can persist for up to a week and the fever rarely rises above 38 °C (100.4 °F). The rash of German measles is typically pink or light red. The rash causes itching and often lasts for about three days. The rash disappears after a few days with no staining or peeling of the skin. When the rash clears up, the skin might shed in very small flakes where the rash covered it. Forchheimer's sign occurs in 20% of cases, and is characterized by small, red papules on the area of the soft palate.

Rubella can affect anyone of any age and is generally a mild disease, rare in infants or those over the age of 40. The older the person is the more severe the symptoms are likely to be. Up to 60% of older girls or women experience joint pain or arthritic type symptoms with rubella.

In children rubella normally causes symptoms which last two days and include:

- Rash beginning on the face which spreads to the rest of the body.

- Low fever of less than 38.3 °C (101 °F).

- Posterior cervical lymphadenopathy.

In older children and adults additional symptoms may be present including:

- Swollen glands

- Coryza (cold-like symptoms)

- Aching joints (especially in young women)

Rare problems can occur including the following:

- Brain inflammation

- Ear infection

Coryza in rubella may convert to pneumonia, either direct viral pneumonia or secondary bacterial pneumonia, and bronchitis (either viral bronchitis or secondary bacterial bronchitis).

Rubella, also known as German measles or three-day measles, is an infection caused by the rubella virus. This disease is often mild with half of people not realizing that they are infected. A rash may start around two weeks after exposure and last for three days. It usually starts on the face and spreads to the rest of the body. The rash is sometimes itchy and is not as bright as that of measles. Swollen lymph nodes are common and may last a few weeks. A fever, sore throat, and fatigue may also occur. In adults joint pain is common. Complications may include bleeding problems, testicular swelling, and inflammation of nerves. Infection during early pregnancy may result in a child born with congenital rubella syndrome (CRS) or miscarriage. Symptoms of CRS include problems with the eyes such as cataracts, ears such as deafness, heart, and brain. Problems are rare after the 20th week of pregnancy.

Rubella is usually spread through the air via coughs of people who are infected. People are infectious during the week before and after the appearance of the rash. Babies with CRS may spread the virus for more than a year. Only humans are infected. Insects do not spread the disease. Once recovered, people are immune to future infections. Testing is available that can verify immunity. Diagnosis is confirmed by finding the virus in the blood, throat, or urine. Testing the blood for antibodies may also be useful.

Rubella is preventable with the rubella vaccine with a single dose being more than 95% effective. Often it is given in combination with the measles vaccine and mumps vaccine, known as the MMR vaccine. When some, but less than 80% of the people are vaccinated, more women might make it to childbearing age without developing immunity by infection or vaccination and CRS rates could increase. Once infected there is no specific treatment.

Rubella is a common infection in many areas of the world. Each year about 100,000 cases of congenital rubella syndrome occur. Rates of disease have decreased in many areas as a result of vaccination. There are ongoing efforts to eliminate the disease globally. In April 2015 the World Health Organization declared the Americas free of rubella transmission. The name "rubella" is from Latin and means "little red". It was first described as a separate disease by German physicians in 1814 resulting in the name "German measles."

Mumps is usually preceded by a set of prodromal symptoms including low-grade fever, headache, and malaise. This is followed by progressive swelling of one or both parotid glands. Parotid gland swelling usually lasts about one week. Other symptoms of mumps can include dry mouth, sore face and/or ears and some patients find it difficult to talk. A vaccine has been available since the 1960s.

Mumps is a viral disease caused by the mumps virus. Initial signs and symptoms often include fever, muscle pain, headache, and feeling tired. This is then usually followed by painful swelling of one or both parotid salivary glands. Symptoms typically occur 16 to 18 days after exposure and resolve after seven to ten days. Symptoms in adults are often more severe than in children. About a third of people have mild or no symptoms. Complications may include meningitis (15 percent), pancreatitis (four percent), permanent deafness, and testicular inflammation which uncommonly results in infertility. Women may develop ovarian swelling but this does not increase the risk of infertility.

Mumps is highly contagious and spreads rapidly among people living in close quarters. The virus is transmitted by respiratory droplets or direct contact with an infected person. Only humans get and spread the disease. People are infectious to each other from about seven days before the start of symptoms to about eight days after. Once an infection has run its course, a person is typically immune for life. Reinfection is possible but the ensuing infection tends to be mild. Diagnosis is usually suspected due to parotid swelling and can be confirmed by isolating the virus on a swab of the parotid duct. Testing for IgM antibodies in the blood is simple and may be useful; however, it can be falsely negative in those who have been immunized.

Mumps is preventable by two doses of the mumps vaccine. Most of the developed world includes it in their immunization programs, often in combination with measles, rubella, and varicella vaccine. Countries that have low immunization rates may see an increase in cases among older age groups and thus worse outcomes. There is no specific treatment. Efforts involve controlling symptoms with pain medication such as paracetamol (acetaminophen). Intravenous immunoglobulin may be useful in certain complications. Hospitalization may be required if meningitis or pancreatitis develops. About one per ten thousand people who are infected die.

Without immunization about 0.1 percent to one percent of the population are affected per year. Widespread vaccination has resulted in a more than 90 percent decline in rates of disease. Mumps is more common in the developing world where vaccination is less common. Outbreaks, however, may still occur in a vaccinated population. Before the introduction of a vaccine, mumps was a common childhood disease worldwide. Larger outbreaks of disease would typically occur every two to five years. Children between the ages of five and nine were most commonly affected. Among immunized populations often those in their early 20s are affected. Around the equator it often occurs all year round while in the more northerly and southerly regions of the world it is more common in the winter and spring. Painful swelling of the parotid glands and testicles was described by Hippocrates in the 5th century BCE.

Roseola typically affects children between six months and two years of age, and begins with a sudden high fever (39–40 °C; 102.2-104 °F). In rare cases, this can cause cause febrile convulsions (also known as febrile seizures or "fever fits") due to the sudden rise in body temperature, but in many cases the child appears normal. After a few days the fever subsides, and just as the child appears to be recovering, a red rash appears. This usually begins on the trunk and then spreads to the arms, legs, and neck. The rash is not itchy and may last 1 to 2 days. In contrast, a child suffering from measles would usually appear sicker, with symptoms of conjunctivitis, cold-like symptoms, and a cough, and their rash would affect the face and last for several days. Liver dysfunction can occur in rare cases.

A small percentage of children acquire HHV-6 with few sign or symptoms of the disease. Exanthema subitum occurs in approximately 30% of children during primary HHV-6 infection. Others may show symptoms significant enough that other more serious infections, such as meningitis or measles should be ruled out. In case of febrile seizures, medical advice can be sought for reassurance. However, febrile seizures are not harmful, do not require treatment, and have no long term negative effects unless they last longer than five minutes.

In rare cases, HHV-6 can become active in an adult previously infected during childhood and can show signs of mononucleosis.

A "vaccine-preventable disease" is an infectious disease for which an effective preventive vaccine exists. If a person acquires a vaccine-preventable disease and dies from it, the death is considered a vaccine-preventable death.

The most common and serious vaccine-preventable diseases tracked by the World Health Organization (WHO) are: diphtheria, "Haemophilus influenzae" serotype b infection, hepatitis B, measles, meningitis, mumps, pertussis, poliomyelitis, rubella, tetanus, tuberculosis, and yellow fever. The WHO reports licensed vaccines being available to prevent, or contribute to the prevention and control of, 25 vaccine-preventable infections.

In dogs, signs of distemper vary widely from no signs, to mild respiratory signs indistinguishable from kennel cough, to severe pneumonia with vomiting, bloody diarrhea and death.

Commonly observed signs are a runny nose, vomiting and diarrhea, dehydration, excessive salivation, coughing and/or labored breathing, loss of appetite, and weight loss. If neurological signs develop, incontinence may ensue. Central nervous system signs include a localized involuntary twitching of muscles or groups of muscles, seizures with salivation and jaw movements commonly described as "chewing gum fits", or more appropriately as "distemper myoclonus". As the condition progresses, the seizures worsen and advance to grand mal convulsions followed by death of the animal. The animal may also show signs of sensitivity to light, incoordination, circling, increased sensitivity to sensory stimuli such as pain or touch, and deterioration of motor capabilities. Less commonly, they may lead to blindness and paralysis. The length of the systemic disease may be as short as 10 days, or the start of neurological signs may not come until several weeks or months later. Those few that survive usually have a small tic or twitch of varying levels of severity. With time, this tic will usually diminish somewhat in its severity.

A dog that survives distemper will continue to have both nonlife-threatening and life-threatening signs throughout its lifespan. The most prevalent nonlife-threatening symptom is hard pad disease. This occurs when a dog experiences the thickening of the skin on the pads of its paws as well as on the end of its nose. Another lasting symptom commonly is enamel hypoplasia. Puppies, especially, will have damage to the enamel of teeth that are not completely formed or those that have not yet grown through the gums. This is a result of the virus's killing the cells responsible for manufacturing the tooth enamel. These affected teeth tend to erode quickly.

Life-threatening signs usually include those due to the degeneration of the nervous system. Dogs that have been infected with distemper tend to suffer a progressive deterioration of mental abilities and motor skills. With time, the dog can acquire more severe seizures, paralysis, reduction in sight and incoordination. These dogs are usually humanely euthanized because of the immense pain and suffering they face.

Roseola is an infectious disease caused by certain types of virus. Most infections occur before the age of three. Symptoms vary from absent to the classic presentation of a fever of rapid onset followed by a rash. The fever generally lasts for three to five days. The rash is generally pink and lasts for less than three days. Complications may include febrile seizures, with serious complications being rare.

It is caused by either "human herpesvirus 6" (HHV-6) or "human herpesvirus 7" (HHV-7). Spread is usually through the saliva of those who are otherwise healthy. However, it may also spread from the mother to baby during pregnancy. Diagnosis is typically based on symptoms but can be confirmed with blood tests. Low numbers of white blood cells may also be present.

Treatment includes sufficient fluids and medications to treat the fever. Nearly all people are infected at some point in time. Males and females are affected equally often. The disease was first described in 1910 while the causal virus was determined in 1988. The disease may reactivate in those with a weakened immune system and may result in significant health problems.

Non-specific effects of vaccines (also called "heterologous effects" or "off-target effects") are effects which go beyond the specific protective effects against the targeted diseases. Non-specific effects can be strongly beneficial, increasing protection against non-targeted infections, but also at times negative, increasing susceptibility to non-targeted infections. This depends on both the vaccine and the sex of the infant.

All live attenuated vaccines studied so far (BCG vaccine, measles vaccine, oral polio vaccine, smallpox vaccine) have been shown to reduce mortality more than can be explained by prevention of the targeted infections. In contrast, inactivated vaccines (diphtheria-tetanus-pertussis vaccine (DTP), hepatitis B vaccine, inactivated polio vaccine) may increase overall mortality despite providing protection against the target diseases.

These effects may be long-lasting, at least up to the time point where a new type of vaccine is given. The non-specific effects can be very pronounced, with significant effects on overall mortality and morbidity. In a situation with herd immunity to the target disease, the non-specific effects can be more important for overall health than the specific vaccine effects.

The non-specific effects should not be confused with the side effects of vaccines (such as local reactions at the side of vaccination or general reactions such as fever, head ache or rash, which usually resolve within days to weeks – or in rare cases anaphylaxis). Rather, non-specific effects represent a form of general immunomodulation, with important consequences for the immune system's ability to handle subsequent challenges.

It is estimated that millions of child deaths in low income countries could be prevented every year if the non-specific effects of vaccines were taken into consideration in immunization programs.

Influenza-like illness (ILI), also known as acute respiratory infection (ARI) and flu-like syndrome/symptoms, is a medical diagnosis of "possible" influenza or other illness causing a set of common symptoms.

Symptoms commonly include fever, shivering, chills, malaise, dry cough, loss of appetite, body aches, and nausea, typically in connection with a sudden onset of illness. In most cases, the symptoms are caused by cytokines released by immune system activation, and are thus relatively non-specific.

Common causes of ILI include the common cold and influenza, which tends to be less common but more severe than the common cold. Less-common causes include side effects of many drugs and manifestations of many other diseases.

In 2012, the World Health Organization estimated that vaccination prevents 2.5 million deaths each year. If there is 100% immunization, and 100% efficacy of the vaccines, one out of seven deaths among young children could be prevented, mostly in developing countries, making this an important global health issue. Four diseases were responsible for 98% of vaccine-preventable deaths: measles, "Haemophilus influenzae" serotype b, pertussis, and neonatal tetanus.

The Immunization Surveillance, Assessment and Monitoring program of the WHO monitors and assesses the safety and effectiveness of programs and vaccines at reducing illness and deaths from diseases that could be prevented by vaccines.

Vaccine-preventable deaths are usually caused by a failure to obtain the vaccine in a timely manner. This may be due to financial constraints or to lack of access to the vaccine. A vaccine that is generally recommended may be medically inappropriate for a small number of people due to severe allergies or a damaged immune system. In addition, a vaccine against a given disease may not be recommended for general use in a given country, or may be recommended only to certain populations, such as young children or older adults. Every country makes its own vaccination recommendations, based on the diseases that are common in its area and its healthcare priorities. If a vaccine-preventable disease is uncommon in a country, then residents of that country are unlikely to receive a vaccine against it. For example, residents of Canada and the United States do not routinely receive vaccines against yellow fever, which leaves them vulnerable to infection if travelling to areas where risk of yellow fever is highest (endemic or transitional regions).

The causes of influenza-like illness range from benign self-limited illnesses such as gastroenteritis, rhinoviral disease, and influenza, to severe, sometimes life-threatening, diseases such as meningitis, sepsis, and leukemia.

Viral cardiomyopathy occurs when viral infections cause myocarditis with a resulting thickening of the myocardium and dilation of the ventricles. These viruses include Coxsackie B and adenovirus, echoviruses, influenza H1N1, Epstein-Barr virus, rubella (German measles virus), varicella (chickenpox virus), mumps, measles, parvoviruses, yellow fever, dengue fever, polio, rabies and the viruses that cause hepatitis A and C.

Symptoms of viral pneumonia include fever, non-productive cough, runny nose, and systemic symptoms (e.g. myalgia, headache). Different viruses cause different symptoms.

Viral pneumonia is a pneumonia caused by a virus.

Viruses are one of the two major causes of pneumonia, the other being bacteria; less common causes are fungi and parasites. Viruses are the most common cause of pneumonia in children, while in adults bacteria are a more common cause.

Hemorrhagic smallpox, sometimes called bloody pox, fulminant smallpox, and blackpox, is a severe and rare form of smallpox and is usually fatal. Like all forms of smallpox it is caused by the variola virus. It is characterized by an incubation period of 7 to 14 days. It has two stages, the first begins with fever, headache, chills, nausea, vomiting and severe muscle aches. The skin flushes in a deep-purple, uneven pattern across the face. The early stage is often mistaken for measles. The late stage is characterized by the appearance of small blisters resembling a severe form of chickenpox. These small blisters then flatten until they are even with the skin, and change into reddish lesions similar to those seen in measles. The skin then turns a deep purple. Lesions appear inside the mouth and active bleeding from oral and nasal mucous membranes is common. This is followed by active bleeding in the gastrointestinal tract, and blood appears in the stool and urine. Blood studies resemble the clinical values of disseminated intravascular coagulation.

Characteristics of a viral infection can include pain, swelling, redness, impaired function, fever, drowsiness, confusion and convulsions.

Meningitis is a very common in children. Newborns can develop herpes virus infections through contact with infected secretions in the birth canal. Other viral infections are acquired by breathing air contaminated with virus-containing droplets exhaled by an infected person. Arbovirus infections are acquired from bites by infected insects (called epidemic encephalitis). Viral central nervous system infections in newborns and infants usually begin with fever. The inability of infants to communicate directly makes it difficult to understand their symptoms. Newborns may have no other symptoms and may initially not otherwise appear ill. Infants older than a month or so typically become irritable and fussy and refuse to eat. Vomiting is common. Sometimes the soft spot on top of a newborn's head (fontanelle) bulges, indicating an increase in pressure on the brain. Because irritation of the meninges is worsened by movement, an infant with meningitis may cry more, rather than calm down, when picked up and rocked. Some infants develop a strange, high-pitched cry. Infants with encephalitis often have seizures or other abnormal movements. Infants with severe encephalitis may become lethargic and comatose and then die. To make the diagnosis of meningitis or the diagnosis of encephalitis, doctors do a spinal tap (lumbar puncture) to obtain cerebrospinal fluid (CSF) for laboratory analysis in children.

Viral meningitis characteristically presents with fever, headache and neck stiffness. Fever is the result of cytokines released that affect the thermoregulatory neurons of the hypothalamus. Cytokines and increased intracranial pressure stimulate nociceptors in the brain that lead to headaches. Neck stiffness is the result of inflamed meninges stretching due to flexion of the spine. In contrast to bacterial meningitis, symptoms are often less severe and do not progress as quickly. Nausea, vomiting and photophobia (light sensitivity) also commonly occur, as do general signs of a viral infection, such as muscle aches and malaise. Increased cranial pressure from viral meningitis stimulates the area postrema, which causes nausea and vomiting. Photophobia is due to meningeal irritation. In severe cases, people may experience concomitant encephalitis (meningoencephalitis), which is suggested by symptoms such as altered mental status, seizures or focal neurologic deficits.

Babies with viral meningitis may only appear irritable, sleepy or have trouble eating. In severe cases, people may experience concomitant encephalitis (meningoencephalitis), which is suggested by symptoms such as altered mental status, seizures or focal neurologic deficits. The pediatric population may show some additional signs and symptoms that include jaundice and bulging fontanelles.

An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

Viral meningitis, also known as aseptic meningitis, is a type of meningitis due to a viral infection. It results in inflammation of the meninges (the membranes covering the brain and spinal cord). Symptoms commonly include headache, fever, sensitivity to light, and neck stiffness.

Viruses are the most common cause of aseptic meningitis. Most cases of viral meningitis are caused by enteroviruses (common stomach viruses). However, other viruses can also cause viral meningitis. For instance, West Nile virus, mumps, measles, herpes simplex types I and II, varicella, and lymphocytic choriomeningitis (LCM) virus. Based on clinical symptoms, viral meningitis cannot be reliably differentiated from bacterial meningitis, although viral meningitis typically follows a more benign clinical course. Viral meningitis has no evidence of bacteria present in cerebral spinal fluid (CSF). Therefore, lumbar puncture with CSF analysis is often needed to identify the disease.

In most causes there is no specific treatment, with efforts generally aimed at relieving symptoms (headache, fever, or nausea). A few viral causes, such as HSV, have specific treatments.

In the United States viral meningitis is the cause of greater than half of all cases of meningitis. From 1988–1999, about 36,000 cases occurred a year. While the disease can occur in both children and adults it is more common in children.

Symptoms of endemic typhus include headache, fever, muscle pain, joint pain, nausea and vomiting. 40–50% of patients will develop a discrete rash six days after the onset of signs. Up to 45% will develop neurological signs such as confusion, stupor, seizures or imbalance.

Symptoms may resemble those of measles, rubella, or possibly Rocky Mountain spotted fever. These symptoms are likely caused by a vasculitis caused by the rickettsia.