Zika virus infections have been strongly associated with Guillain–Barré syndrome (GBS), which is a rapid onset of muscle weakness caused by the immune system damaging the peripheral nervous system, and which can progress to paralysis. While both GBS and Zika infection can simultaneously occur in the same individual, it is difficult to definitively identify Zika virus as the cause of GBS. Several countries affected by Zika outbreaks have reported increases in the rate of new cases of GBS. During the 2013–2014 outbreak in French Polynesia there were 42 reported cases of GBS over a 3-month period, compared to between 3 and 10 annually prior to the outbreak.

Most people who are infected have no or few symptoms. Otherwise the most common signs and symptoms of Zika fever are fever, rash, conjunctivitis (red eyes), muscle and joint pain, and headache, which are similar to signs and symptoms of dengue and chikungunya fever. The time from a mosquito bite to developing symptoms is not yet known, but is probably a few days to a week. The disease lasts for several days to a week and is usually mild enough that people do not have to go to a hospital.

Due to being in the same family as dengue, there has been concern that it could cause similar bleeding disorders. However that has only been documented in one case, with blood seen in semen, also known as hematospermia.

Though caused by different infections, the signs and symptoms of TORCH syndrome are consistent. They include hepatosplenomegaly (enlargement of the liver and spleen), fever, lethargy, difficulty feeding, anemia, petechiae, purpurae, jaundice, and chorioretinitis. The specific infection may cause additional symptoms.

TORCH syndrome may develop before birth, causing stillbirth, in the neonatal period, or later in life.

The signs and symptoms of a vertically transmitted infection depend on the individual pathogen. It may cause subtle signs such as a influenza-like illness and may not even be noticed by the mother during the pregnancy. In such cases, the effects may be seen first at birth.

Symptoms of a vertically transmitted infection may include fever and flu like symptoms. The newborn is often small for gestational age. A petechial rash on the skin may be present, with small reddish or purplish spots due to bleeding from capillaries under the skin. An enlarged liver and spleen (hepatosplenomegaly) is common, as is jaundice. However, jaundice is less common in hepatitis B because a newborn's immune system is not developed well enough to mount a response against liver cells, as would normally be the cause of jaundice in an older child or adult. Hearing impairment, eye problems, mental retardation, autism, and death can be caused by vertically transmitted infections. The mother often has a mild infection with few or no symptoms.

The genetic conditions of Aicardi-Goutieres syndrome are possibly present in a similar manner.

TORCH syndrome is caused by in utero infection with one of the TORCH agents, disrupting fetal development.

A subclinical infection (sometimes called a preinfection) is an infection that, being , is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals. An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

A vertically transmitted infection is an infection caused by pathogens (such as bacteria and viruses) that uses mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It can occur when the mother gets an infection as an intercurrent disease in pregnancy. Nutritional deficiencies may exacerbate the risks of perinatal infection.

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.

BVDV infection has a wide manifestation of clinical signs including fertility issues, milk drop, pyrexia, diarrhoea and fetal infection. Occasionally, a severe acute form of BVD may occur. These outbreaks are characterized by thrombocytopenia with high morbidity and mortality. However, clinical signs are frequently mild and infection insidious, recognised only by BVDV’s immunosuppressive effects perpetuating other circulating infectious diseases (particularly scours and pneumonias).

The incubation period of the chikungunya virus ranges from one to twelve days, and is most typically three to seven. The disease may be asymptomatic, but generally is not, as 72% to 97% of those infected will develop symptoms. Characteristic symptoms include sudden onset with high fever, joint pain, and rash. Other symptoms may occur, including headache, fatigue, digestive complaints, and conjunctivitis.

Information gained during recent epidemics suggests that chikungunya fever may result in a chronic phase as well as the phase of acute illness. Within the acute phase, two stages have been identified: a viral stage during the first five to seven days, during which viremia occurs, followed by a convalescent stage lasting approximately ten days, during which symptoms improve and the virus cannot be detected in the blood. Typically, the disease begins with a sudden high fever that lasts from a few days to a week, and sometimes up to ten days. The fever is usually above and sometimes reaching and may be biphasic—lasting several days, breaking, and then returning. Fever occurs with the onset of viremia, and the level of virus in the blood correlates with the intensity of symptoms in the acute phase. When IgM, an antibody that is a response to the initial exposure to an antigen, appears in the blood, viremia begins to diminish. However, headache, insomnia and an extreme degree of exhaustion remain, usually about five to seven days.

Following the fever, strong joint pain or stiffness occurs; it usually lasts weeks or months, but may last for years. The joint pain can be debilitating, often resulting in near immobility of the affected joints. Joint pain is reported in 87–98% of cases, and nearly always occurs in more than one joint, though joint swelling is uncommon. Typically the affected joints are located in both arms and legs, and are affected symmetrically. Joints are more likely to be affected if they have previously been damaged by disorders such as arthritis. Pain most commonly occurs in peripheral joints, such as the wrists, ankles, and joints of the hands and feet as well as some of the larger joints, typically the shoulders, elbows and knees. Pain may also occur in the muscles or ligaments.

Rash occurs in 40–50% of cases, generally as a maculopapular rash occurring two to five days after onset of symptoms. Digestive symptoms, including abdominal pain, nausea, vomiting or diarrhea, may also occur. In more than half of cases, normal activity is limited by significant fatigue and pain. Infrequently, inflammation of the eyes may occur in the form of iridocyclitis, or uveitis, and retinal lesions may occur.

Temporary damage to the liver may occur.

Rarely, neurological disorders have been reported in association with chikungunya virus, including Guillain–Barré syndrome, palsies, meningoencephalitis, flaccid paralysis and neuropathy. In contrast to dengue fever, Chikungunya fever very rarely causes hemorrhagic complications. Symptoms of bleeding should lead to consideration of alternative diagnoses or co-infection with dengue fever or coexisting congestive hepatopathy.

Bovine viral diarrhea (BVD) or bovine viral diarrhoea (UK English), and previously referred to as bovine virus diarrhoea (BVD), is a significant economic disease of cattle that is endemic in the majority of countries throughout the world. The causative agent, bovine viral diarrhea virus (BVDV), is a member of the "Pestivirus" genus of the family Flaviviridae.

BVD infection results in a wide variety of clinical signs, due to its immunosuppressive effects, as well as having a direct effect on respiratory disease and fertility. In addition, BVD infection of a susceptible dam during a certain period of gestation can result in the production of a persistently infected (PI) fetus.

PI animals recognise intra-cellular BVD viral particles as ‘self’ and shed virus in large quantities throughout life; they represent the cornerstone of the success of BVD as a disease.

An individual may only develop signs of an infection after a period of subclinical infection, a duration that is called the incubation period. This is the case, for example, for subclinical sexually transmitted diseases such as AIDS and genital warts. Individuals with such subclinical infections, and those that never develop overt illness, creates a reserve of individuals that can transmit an infectious agent to infect other individuals. Because such cases of infections do not come to clinical attention, health statistics can often fail to measure the true prevalence of an infection in a population, and this prevents the accurate modeling of its infectious transmission.

Acute toxoplasmosis is often asymptomatic in healthy adults. However, symptoms may manifest and are often influenza-like: swollen lymph nodes, headaches, fever, and fatigue, or muscle aches and pains that last for a month or more. Rarely will a human with a fully functioning immune system develop severe symptoms following infection. People with weakened immune systems are likely to experience headache, confusion, poor coordination, seizures, lung problems that may resemble tuberculosis or Pneumocystis jiroveci pneumonia (a common opportunistic infection that occurs in people with AIDS), or blurred vision caused by severe inflammation of the retina (ocular toxoplasmosis) Young children and immunocompromised people, such as those with HIV/AIDS, those taking certain types of chemotherapy, or those who have recently received an organ transplant, may develop severe toxoplasmosis. This can cause damage to the brain (encephalitis) or the eyes (necrotizing retinochoroiditis). Infants infected via placental transmission may be born with either of these problems, or with nasal malformations, although these complications are rare in newborns. The toxoplasmic trophozoites causing acute toxoplasmosis are referred to as tachyzoites, and are typically found in bodily fluids.

Swollen lymph nodes are commonly found in the neck or under the chin, followed by the armpits and the groin. Swelling may occur at different times after the initial infection, persist, and recur for various times independently of antiparasitic treatment. It is usually found at single sites in adults, but in children, multiple sites may be more common. Enlarged lymph nodes will resolve within one to two months in 60% of cases. However, a quarter of those affected take two to four months to return to normal, and 8% take four to six months. A substantial number (6%) do not return to normal until much later.

Congenital cytomegalovirus infection refers to a condition where cytomegalovirus is transmitted in the prenatal period.

Human cytomegalovirus is one of the vertically transmitted infections that lead to congenital abnormalities. (Others are: toxoplasmosis, rubella, and herpes simplex. )

Most people infected with the West Nile virus usually do not develop symptoms. However, some individuals can develop cases of severe fatigue, weakness, headaches, body aches, joint and muscle pain, vomiting, diarrhea, and rash, which can last for weeks or months. More serious symptoms have a greater risk of appearing in people over 60 years of age, or those suffering from cancer, diabetes, hypertension, and kidney disease.

Dengue fever is mostly characterized by high fever, headaches, joint pain, and rash. However, more severe instances can lead to hemorrhagic fever, internal bleeding, and breathing difficulty, which can be fatal.

Symptoms vary on severity, from mild unnoticeable symptoms to more common symptoms like fever, rash, headache, achy muscle and joints, and conjunctivitis. Symptoms can last several days to weeks, but death resulting from this infection is rare.

Diagnosis of infection is based upon the recovery of the pathogen or pathogens from the typically sterile sites in the mother or the baby. Unfortunately, as many half of pregnant women are asymptomatic with a gonorrhea infection and other sexually transmitted infections. Samples are obtained from urine, blood or cerebrospinal fluid. Diagnosis of infection can also be aided by the use of more nonspecific tests such as determining the total white blood cell count, cytokine levels and other blood tests and signs.

For infants who are infected by their mothers before birth, two potential adverse scenarios exist:

- Generalized infection may occur in the infant, and can cause complications such as low birth weight, microcephaly, seizures, petechial rash similar to the "blueberry muffin" rash of congenital rubella syndrome, and moderate hepatosplenomegaly (with jaundice). Though severe cases can be fatal, with supportive treatment most infants with CMV disease will survive. However, from 80% to 90% will have complications within the first few years of life that may include hearing loss, vision impairment, and varying degrees of learning disability.

- Another 5% to 10% of infants who are infected but without symptoms at birth will subsequently have varying degrees of hearing and mental or coordination problems. The onset of hearing loss can occur at any point during childhood, although commonly within the first decade. It is progressive and can affect both ears.

These risks appear to be almost exclusively associated with women who previously have not been infected with CMV and who are having their first infection with the virus during pregnancy. There appears to be little risk of CMV-related complications for women who have been infected at least 6 months prior to conception. For this group, which makes up 50% to 80% of the women of child-bearing age, the rate of newborn CMV infection is 1%, and these infants appear to have no significant illness or abnormalities.

The virus can also be transmitted to the infant at delivery from contact with genital secretions or later in infancy through breast milk. However, these infections usually result in little or no clinical illness in the infant.

To summarise, during a pregnancy when a woman who has never had CMV infection becomes infected with CMV, there is a risk that after birth the infant may have CMV-related complications, the most common of which are associated with hearing loss, visual impairment, or diminished mental and motor capabilities. On the other hand, healthy infants and children who acquire CMV after birth have few, if any, symptoms or complications. However, preterm born infants infected with CMV after birth (especially via breastmilk). This can lead to cognitive and motor impairments later in life.

Congenital cytomegalovirus infection can be an important cause of intraventricular hemorrhage and neonatal encephalopathy.

Neonatal infections are infections of the neonate (newborn) during the neonatal period or first four weeks after birth. Neonatal infections may be contracted by transplacental transfer in utero, in the birth canal during delivery (perinatal), or by other means after birth. Some neonatal infections are apparent soon after delivery, while others may develop postpartum within the first week or month. Some infections acquired in the neonatal period do not become apparent until much later such as HIV, hepatitis B and malaria.

There is a higher risk of infection for preterm or low birth weight neonates. Respiratory tract infections contracted by preterm neonates may continue into childhood or possibly adulthood with long-term effects that limit one's ability to engage in normal physical activities, decreasing one's quality of life and increasing health care costs. In some instances, neonatal respiratory tract infections may increase one's susceptibility to future respiratory infections and inflammatory responses related to lung disease.

Antibiotics can be effective treatments for neonatal infections, especially when the pathogen is quickly identified. Instead of relying solely on culturing techniques, pathogen identification has improved substantially with advancing technology; however, neonate mortality has not kept pace and remains 20% to 50%. While preterm neonates are at a particularly high risk, full term and post-term infants can also develop infection. Neonatal infection may also be associated with premature rupture of membranes (breakage of the amniotic sac) which substantially increases the risk of neonatal sepsis by allowing passage for bacteria to enter the womb prior to the birth of the infant. Neonatal infection can be distressing to the family and it initiates concentrated effort to treat it by clinicians.Research to improve treatment of infections and prophylactic treatment of the mother to avoid infections of the infant is ongoing.

Chikungunya is an infection caused by the chikungunya virus (CHIKV). Symptoms include fever and joint pain. These typically occur two to twelve days after exposure. Other symptoms may include headache, muscle pain, joint swelling, and a rash. Most people are better within a week; however, occasionally the joint pain may last for months. The risk of death is around 1 in 1,000. The very young, old, and those with other health problems are at risk of more severe disease.

The virus is spread between people by two types of mosquitos: "Aedes albopictus" and "Aedes aegypti". They mainly bite during the day. The virus may circulate within a number of animals including birds and rodents. Diagnosis is by either testing the blood for the virus's RNA or antibodies to the virus. The symptoms can be mistaken for those of dengue fever and Zika fever. After a single infection it is believed most people become immune.

The best means of prevention is overall mosquito control and the avoidance of bites in areas where the disease is common. This may be partly achieved by decreasing mosquitoes' access to water and with the use of insect repellent and mosquito nets. There is no vaccine and no specific treatment as of 2016. Recommendations include rest, fluids, and medications to help with fever and joint pain.

While the disease typically occurs in Africa and Asia, outbreaks have been reported in Europe and the Americas since the 2000s. In 2014 more than a million suspected cases occurred. In 2014 it was occurring in Florida in the continental United States but as of 2016 there was no further locally acquired cases. The disease was first identified in 1952 in Tanzania. The term is from the Kimakonde language and means "to become contorted".

Feline zoonosis are the viral, bacterial, fungal, protozoan, nematode and arthropod infections that can be transmitted to humans from the domesticated cat, "Felis catus". Some of these are diseases are reemerging and newly emerging infections or infestations caused by zoonotic pathogens transmitted by cats. In some instances, the cat can display symptoms of infection (these may differ from the symptoms in humans) and sometimes the cat remains asymptomatic. There can be serious illnesses and clinical manifestations in people who become infected. This is dependent on the immune status and age of the person. Those who live in close association with cats are more prone to these infections. But those that do not keep cats as pets are also able to acquire these infections because of the transmission can be from cat feces and the parasites that leave their bodies.

People can acquire cat-associated infections through bites, scratches or other direct contact of the skin or mucous membranes with the cat. This includes 'kissing' or letting the animal lick the mouth or nose. Mucous membranes are easily infected when the pathogen is in the mouth of the cat. Pathogens can also infect people when there is contact with animal saliva, urine and other body fluids or secretions, When fecal material is unintentionally ingested, infection can occur. Feline zooinosis can be acquired by a person by inhalation of aerosols or droplets coughed up by the cat.

In the United States, forty percent of homes have at least one cat. Some contagious infections such as campylobacteriosis and salmonellosis cause visible symptoms of the disease in cats. Other infections, such as cat scratch disease and toxoplasmosis, have no visible symptoms and are carried by apparently healthy cats.

Herpes simplex is divided into two types; HSV-1 causes primarily mouth, throat, face, eye, and central nervous system infections, whereas HSV-2 causes primarily anogenital infections. However, each may cause infections in all areas.

Neonatal herpes simplex is a HSV infection in an infant. It is a rare but serious condition, usually caused by vertical transmission of HSV-1 or -2) from mother to newborn. During immunodeficiency, herpes simplex can cause unusual lesions in the skin. One of the most striking is the appearance of clean linear erosions in skin creases, with the appearance of a knife cut. Herpetic sycosis is a recurrent or initial herpes simplex infection affecting primarily the hair follicles. Eczema herpeticum is an infection with herpesvirus in patients with chronic atopic dermatitis may result in spread of herpes simples throughout the eczematous areas.

Herpetic keratoconjunctivitis, a primary infection, typically presents as swelling of the conjunctiva and eyelids (blepharoconjunctivitis), accompanied by small white itchy lesions on the surface of the cornea.

Herpetic sycosis is a recurrent or initial herpes simplex infection affecting primarily the hair follicle.

Toxoplasmosis is a parasitic disease caused by "Toxoplasma gondii". Infections with toxoplasmosis usually cause no obvious symptoms in adults. Occasionally there may be a few weeks or months of mild flu-like illness such as muscle aches and tender lymph nodes. In a small number of people, eye problems may develop. In those with a weak immune system, severe symptoms such as seizures and poor coordination may occur. If infected during pregnancy, a condition known as congenital toxoplasmosis may affect the child.

Toxoplasmosis is usually spread by eating poorly cooked food that contains cysts, exposure to infected cat feces, and from a mother to a child during pregnancy if the mother becomes infected. Rarely the disease may be spread by blood transfusion. It is not otherwise spread between people. The parasite is only known to reproduce sexually in the cat family. However, it can infect most types of warm-blooded animals, including humans. Diagnosis is typically by testing blood for antibodies or by testing amniotic fluid for the parasite's DNA.

Prevention is by properly preparing and cooking food. It is also recommended that pregnant women do not clean cat litter boxes. Treatment of otherwise healthy people is usually not needed. During pregnancy spiramycin or pyrimethamine/sulfadiazine and folinic acid may be used for treatment.

Up to half of the world's population is infected by toxoplasmosis but have no symptoms. In the United States about 23% are affected and in some areas of the world this is up to 95%. About 200,000 cases of congenital toxoplasmosis occur a year. Charles Nicolle and Louis Manceaux first described the organism in 1908. In 1941 transmission during pregnancy from a mother to a child was confirmed.

Respiratory infection is usually asymptomatic in pigs more than 2 months old, but it can cause abortion, high mortality in piglets, and coughing, sneezing, fever, constipation, depression, seizures, ataxia, circling, and excess salivation in piglets and mature pigs. Mortality in piglets less than one month of age is close to 100%, but it is less than 10% in pigs between one and six months of age. Pregnant swine can reabsorb their litters or deliver mummified, stillborn, or weakened piglets. In cattle (see next section), symptoms include intense itching followed by neurological signs and death. In dogs, symptoms include intense itching, jaw and pharyngeal paralysis, howling, and death Any infected secondary host generally only lives two to three days.

Genital infection appears to have been common in a great part of the 20th century in many European countries in swine herds, where boars from boar centres were used for natural service of sows or gilts. This disease manifestation has always been asymptomatic in affected pigs, and presence of the infection on a farm was detected only because of cases in cattle showing pruritus on the hindquarters (vaginal infection, see below).

In susceptible animals other than swine, infection is usually fatal, and the affected animals most often show intense pruritus in a skin area.

Pruritus in Aujeszky's disease is considered a phantom sensation, and virus has never been found at the site of pruritus.