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).

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.

Apart from respiratory involvement, illnesses and presentations of adenovirus include gastroenteritis, conjunctivitis, cystitis, and rash illness. Symptoms of respiratory illness caused by adenovirus infection range from the common cold syndrome to pneumonia, croup, and bronchitis. Patients with compromised immune systems are especially susceptible to severe complications of adenovirus infection. Acute respiratory disease (ARD), first recognized among military recruits during World War II, can be caused by adenovirus infections during conditions of crowding and stress.

"Pharyngoconjunctival fever" is a specific presentation of adenovirus infection, manifested as:

- high fever that lasts 4–5 days

- pharyngitis (sore throat)

- conjunctivitis (inflamed eyes, usually without pus formation like pink eye)

- enlargement of the lymph nodes of the neck

- headache, malaise, and weakness

- Incubation period of 5–9 days

It usually occurs in the age group 5–18. It is often found in summer camps and during the spring and fall in schools. In Japan, the illness is commonly referred to as "pool fever" as it is often spread via public swimming pools.

Initial signs of FVR include coughing, sneezing, nasal discharge, conjunctivitis, and sometimes fever (up to 106) and loss of appetite. These usually resolve within four to seven days, but secondary bacterial infections can cause the persistence of clinical signs for weeks. Frontal sinusitis and empyema can also result.

FHV-1 also has a predilection for corneal epithelium, resulting in corneal ulcers, often pinpoint or dendritic in shape. Other ocular signs of FHV-1 infection include conjunctivitis, keratitis, keratoconjunctivitis sicca (decreased tear production), and corneal sequestra. Infection of the nasolacrimal duct can result in chronic epiphora (excess tearing). Ulcerative skin disease can also result from FHV-1 infection. FHV-1 can also cause abortion in pregnant queens, usually at the sixth week of gestation, although this may be due to systemic effects of the infection rather than the virus directly.

In chronic nasal and sinus disease of cats, FHV-1 may play more of an initiating role than an ongoing cause. Infection at an early age may permanently damage nasal and sinus tissue, causing a disruption of ciliary clearance of mucus and bacteria, and predispose these cats to chronic bacterial infections.

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.

Feline viral rhinotracheitis (FVR) is an upper respiratory or pulmonary infection of cats caused by "feline herpesvirus 1", of the family "Herpesviridae". It is also commonly referred to as feline influenza, feline coryza, and feline pneumonia but, as these terms describe other very distinct collections of respiratory symptoms, they are misnomers for the condition. Viral respiratory diseases in cats can be serious, especially in catteries and kennels. Causing one-half of the respiratory diseases in cats, FVR is the most important of these diseases and is found worldwide. The other important cause of feline respiratory disease is "feline calicivirus".

FVR is very contagious and can cause severe disease, including death from pneumonia in young kittens. It can cause flat-chested kitten syndrome, but most evidence for this is anecdotal. All members of the "Felidae" family are susceptible to FVR; in fact, FHV-1 has caused a fatal encephalitis in lions in Germany.

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.

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.

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.

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.

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.

Aujeszky's disease, usually called pseudorabies in the United States, is a viral disease in swine that has been endemic in most parts of the world. It is caused by "Suid herpesvirus 1" (SuHV1). Aujeszky's disease is considered to be the most economically important viral disease of swine in areas where hog cholera has been eradicated. Other mammals, such as humans, cattle, sheep, goats, cats, dogs, and raccoons, are also susceptible. The disease is usually fatal in these animal species bar humans.

The term "pseudorabies" is found inappropriate by many people, as SuHV1 is a herpesvirus and not related to the rabies virus.

Research on SuHV1 in pigs has pioneered animal disease control with genetically modified vaccines. SuHV1 is now used in model studies of basic processes during lytic herpesvirus infection, and for unravelling molecular mechanisms of herpesvirus neurotropism.

Herpes infections usually show no symptoms; when symptoms do appear they typically resolve within two weeks. The main symptom of oral infection is inflammation of the mucosa of the cheek and gums—known as acute herpetic gingivostomatitis—which occurs within 5–10 days of infection. Other symptoms may also develop, including headache, nausea, dizziness and painful ulcers—sometimes confused with canker sores—fever, and sore throat.

Primary HSV infection in adolescents frequently manifests as severe pharyngitis with lesions developing on the cheek and gums. Some individuals develop difficulty in swallowing (dysphagia) and swollen lymph nodes (lymphadenopathy). Primary HSV infections in adults often results in pharyngitis similar to that observed in glandular fever (infectious mononucleosis), but gingivostomatitis is less likely.

Recurrent oral infection is more common with HSV-1 infections than with HSV-2. Symptoms typically progress in a series of eight stages:

1. Latent (weeks to months incident-free): The remission period; After initial infection, the viruses move to sensory nerve ganglia (trigeminal ganglion), where they reside as lifelong, latent viruses. Asymptomatic shedding of contagious virus particles can occur during this stage.

2. Prodromal (day 0–1): Symptoms often precede a recurrence. Symptoms typically begin with tingling (itching) and reddening of the skin around the infected site. This stage can last from a few days to a few hours preceding the physical manifestation of an infection and is the best time to start treatment.

3. Inflammation (day 1): Virus begins reproducing and infecting cells at the end of the nerve. The healthy cells react to the invasion with swelling and redness displayed as symptoms of infection.

4. Pre-sore (day 2–3): This stage is defined by the appearance of tiny, hard, inflamed papules and vesicles that may itch and are painfully sensitive to touch. In time, these fluid-filled blisters form a cluster on the lip (labial) tissue, the area between the lip and skin (vermilion border), and can occur on the nose, chin, and cheeks.

5. Open lesion (day 4): This is the most painful and contagious of the stages. All the tiny vesicles break open and merge to create one big, open, weeping ulcer. Fluids are slowly discharged from blood vessels and inflamed tissue. This watery discharge is teeming with active viral particles and is highly contagious. Depending on the severity, one may develop a fever and swollen lymph glands under the jaw.

6. Crusting (day 5–8): A honey/golden crust starts to form from the syrupy exudate. This yellowish or brown crust or scab is not made of active virus but from blood serum containing useful proteins such as immunoglobulins. This appears as the healing process begins. The sore is still painful at this stage, but, more painful, however, is the constant cracking of the scab as one moves or stretches their lips, as in smiling or eating. Virus-filled fluid will still ooze out of the sore through any cracks.

7. Healing (day 9–14): New skin begins to form underneath the scab as the virus retreats into latency. A series of scabs will form over the sore (called Meier Complex), each one smaller than the last. During this phase irritation, itching, and some pain are common.

8. Post-scab (12–14 days): A reddish area may linger at the site of viral infection as the destroyed cells are regenerated. Virus shedding can still occur during this stage.

The recurrent infection is thus often called "herpes simplex labialis". Rare reinfections occur inside the mouth ("intraoral HSV stomatitis") affecting the gums, alveolar ridge, hard palate, and the back of the tongue, possibly accompanied by "herpes labialis".

A lesion caused by herpes simplex can occur in the corner of the mouth and be mistaken for angular cheilitis of another cause. Sometimes termed "angular herpes simplex". A cold sore at the corner of the mouth behaves similarly to elsewhere on the lips. Rather than utilizing antifungal creams, angular herpes simplex is treated in the same way as a cold sore, with topical antiviral drugs.

The term "labia" means "lip". Herpes labialis does not refer to the labia of the genitals, though the origin of the word is the same. When the viral infection affects both face and mouth, the broader term "orofacial herpes" is used, whereas the term "herpetic stomatitis" is used to specifically describe infection of the mouth; "stomatitis" is derived from the Greek word stoma that means "mouth".

Some disease-carrying arthropods use cats as a vector, or carrier. Fleas and ticks can carry pathogenic organisms that infect a person with Lyme disease, tick borne encephalitis, and Rocky mountain spotted fever

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.

In very low birth weight infants (VLBWI), systemic fungus infection is a hospital-acquired infection with serious consequences. The pathogens are usually "Candida albicans" and "Candida parapsilosis". A small percentage of fungal infections are caused by "Aspergillus", "Zygomycetes", "Malassezia", and "Trichosporon". Infection is usually late-onset. Up to 9% of VLBWI with birth weights of <1,000 g develop these fungus infections leading to sepsis or meningitis. As many as one-third of these infants can die. Candidiasis is associated with retinopathy, prematurity and negative neurodevelopmental consequences. Candida can colonize the gastrointestinal tract of low birthweight infants (LBI). This gastrointestinal colonization is often a precursor to a more serious invasive infection. The risk of serious candida infection increases when multiple factors are present. These are: thrombocytopenia, the presence of candidal dermatitis, the use of systemic steroids, birth weights of <1,000 g, presence of a central catheter, postponing enteral feeding, vaginal delivery, and the amount of time broad-spectrum antibiotics were given.

Trench fever, also known as five-day fever or quintan fever, is the initial manifestation of "B. quintana" infection. Clinical manifestations range from asymptomatic infection to severe illness. Classical presentations include a febrile illness of acute onset, headache, dizziness, and shin pain. Chronic infection manifestations include attacks of fever and aching in some cases and persistent bacteremia in soldiers and homeless people.

"B. henselae" and "B. quintana" can cause bacillary angiomatosis, a vascular proliferative disease involving mainly the skin, and other organs. The disease was first described in human immunodificiency virus (HIV) patients and organ transplant recipients. Severe, progressive and disseminated disease may occur in HIV patients. Differential diagnoses include Kaposi´s sarcoma, pyogenic granuloma, hemangioma, "verruga Peruana", and subcutanous tumors. Lesions can affect bone marrow, liver, spleen, or lymph nodes.

A list of the more common and well-known diseases associated with infectious pathogens is provided and is not intended to be a complete listing.

A hospital-acquired infection (HAI), also known as a nosocomial infection, is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is sometimes instead called a health care–associated infection (HAI or HCAI). Such an infection can be acquired in hospital, nursing home, rehabilitation facility, outpatient clinic, or other clinical settings. Infection is spread to the susceptible patient in the clinical setting by various means. Health care staff can spread infection, in addition to contaminated equipment, bed linens, or air droplets. The infection can originate from the outside environment, another infected patient, staff that may be infected, or in some cases, the source of the infection cannot be determined. In some cases the microorganism originates from the patient's own skin microbiota, becoming opportunistic after surgery or other procedures that compromise the protective skin barrier. Though the patient may have contracted the infection from their own skin, the infection is still considered nosocomial since it develops in the health care setting.

In the United States, the Centers for Disease Control and Prevention estimated roughly 1.7 million hospital-associated infections, from all types of microorganisms, including bacteria and fungi combined, cause or contribute to 99,000 deaths each year. In Europe, where hospital surveys have been conducted, the category of gram-negative infections are estimated to account for two-thirds of the 25,000 deaths each year. Nosocomial infections can cause severe pneumonia and infections of the urinary tract, bloodstream and other parts of the body. Many types are difficult to treat with antibiotics. In addition, antibiotic resistance can complicate treatment.

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.

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.

Infections associated with diseases are those that are associated with possible infectious etiologies, that meet the requirements of Koch's postulates. Other methods of causation are described by the Bradford Hill criteria and Evidence-based medicine. Koch's postulates have been altered by some epidemiologists based upon sequence-based detection of distinctive pathogenic nucleic acid sequences in tissue samples. Using this method, absolute statements are not always possible regarding causation. Since this is true, higher amounts of distinctive pathogenic nucleic acid sequences would be in those exhibiting disease compared to controls since inoculating those without the pathogen is unethical. In addition, the DNA load should drop or become lower with the resolution of the disease. The distinctive pathogenic nucleic acid sequences load should also increase upon recurrence.

Other conditions are met to establish cause or association including studies in disease transmission. This means that there should be a high disease occurrence in those carrying an pathogen, evidence of a serologicalresponse to the pathogen, and the success of vaccination prevention. Direct visualization of the pathogen, the identification of different strains, immunological responses in the host, how the infection is spread and, the combination of these should all be taken into account to determine the probability that an infectious agent is the cause of the disease. A conclusive determination of a causal role of an infectious agent for in a particular disease using Koch's postulates is desired yet this might not be possible.

The leading cause of death worldwide is cardiovascular disease, but infectious diseases are the second leading cause of death worldwide and the leading cause of death in infants and children.