In an endemic herd, only a minority of the animals develops clinical signs; most animals either eliminate the infection or become asymptomatic carriers. The mortality rate is about 1%, but up to 50% of the animals in the herd can be asymptomatically infected, resulting in losses in production. Once the symptoms appear, paratuberculosis is progressive and affected animals eventually die. The percentage of asymptomatic carriers that develop overt disease is unknown.

Life expectancy with Fabry disease for males was 58.2 years, compared with 74.7 years in the general population, and for females 75.4 years compared with 80.0 years in the general population, according to registry data from 2001 to 2008. The most common cause of death was cardiovascular disease, and most of those had received kidney replacements.

Cytomegalic inclusion body disease (CIBD) is a series of signs and symptoms caused by cytomegalovirus infection, toxoplasmosis or other rare infections such as herpes or rubella viruses. It can produce massive calcification of the central nervous system, and often the kidneys.

Cytomegalic inclusion body disease is the most common cause of congenital abnormalities in the United States. It can also cause pneumonia and other diseases in immunocompromised patients, such as those with HIV/AIDS or recipients of organ transplants.

MAP is capable of causing Johne's-like symptoms in humans, though difficulty in testing for MAP infection presents a diagnostic hurdle.

Clinical similarities are seen between Johne's disease in ruminants and inflammatory bowel disease in humans, and because of this, some researchers contend the organism is a cause of Crohn's disease. However, epidemiologic studies have provided variable results; in certain studies, the organism (or an immune response directed against it) has been much more frequently found in patients with Crohn's disease than asymptomatic people.

Fabry disease is estimated to occur in one in 40,000 to one in 120,000 live births.

Various systems are affected.

- CNS abnormalities – microcephaly, mental retardation, spasticity, epilepsy, periventricular calcification

- Eye – choroidoretinitis and optic atrophy

- Ear – sensorineural deafness

- Liver – hepatosplenomegaly and jaundice due to hepatitis

- Lung – pneumonitis (interstitial pneumonitis)

- Heart – myocarditis

- Thrombocytopenic purpura, haemolytic anaemia

- Late sequelae in individuals asymptomatic at birth – hearing defects and reduced intelligence

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.

Fever and sickness behavior and other signs of infection are often taken to be due to them. However, they are evolved physiological and behavioral responses of the host to clear itself of the infection. Instead of incurring the costs of deploying these evolved responses to infections, the body opts to tolerate an infection as an alternative to seeking to control or remove the infecting pathogen.

Subclinical infections are important since they allow infections to spread from a reserve of carriers. They also can cause clinical problems unrelated to the direct issue of infection. For example, in the case of urinary tract infections in women, this infection may cause preterm delivery if the person becomes pregnant without proper treatment.

Pacheco's disease is an acute and often lethal infectious disease in psittacine birds. The disease is caused by a group of herpesviruses, "Psittacid herpesvirus 1" (PsHV-1), which consists of four genotypes. Birds which do not succumb to Pacheco's disease after infection with the virus become asymptomatic carriers that act as reservoirs of the infection. These persistently infected birds, often Macaws, Amazon parrots and some species of conures, shed the virus in feces and in respiratory and oral secretions. Outbreaks can occur when stress causes healthy birds who carry the virus to shed it. Birds generally become infected after ingesting the virus in contaminated material, and show signs of the disease within several weeks.

The main sign of Pacheco's disease is sudden death, sometimes preceded by a short, severe illness. If a bird survives Pacheco's disease following infection with PsHV-1 genotypes 1, 2 or 3, it may later develop internal papilloma disease in the gastrointestinal tract.

Susceptible parrot species include the African gray parrot, and cockatoo. Native Australian birds, such as the eclectus parrot, Bourke's parrot, and budgerigar are susceptible to Pacheco's disease, although the disease itself has not been found in Australia.

Equine venereal diseases are sexually transmitted infections in horses. They include contagious equine metritis (CEM) (caused by "Taylorella equigenitalis") and equine coital exanthema (caused by equine herpesvirus 3).

There are many different forms on prevention of syphilis and other sexually transmitted diseases in general. Prevention of syphilis includes avoiding contact of bodily fluids with an infected person. This can be particularly difficult because syphilis is usually transmitted by people who are unaware that they have the disease because they do not have any visible sores or rashes that may denote having an infection in general. Being abstinent or having mutually monogamous sex with a person who is uninfected with any type of sexually transmitted disease is the greatest guarantee of not becoming infected with syphilis or any form of a sexually transmitted disease. Using latex condoms can however reduce the risk of obtaining syphilis. In order to prevent further contamination to other individuals, benzathine penicillin is given to any contacts. Washing, douching, or urinating cannot prevent the transmission of a sexually transmitted disease in general.

Individuals obtain syphilis through a variety of circumstances. In general, syphilis can be transmitted from individual to individual through direct contact with sores that are present on the external genitals, vagina, rectum, anus, lips, or mouth. Transmission can occur through any form of sexual contact, including vaginal, anal, and oral sex. In addition, women who are pregnant and infected with syphilis can transmit the disease onto their child as well. If transmission has occurred, it is important to check up immediately with a physician to avoid further damage.

This disease affects the external genitalia, and is caused by equine herpesvirus 3. This disease remains with the horse for all its life. Equine coital exanthema is believed to only be transmitted during the acute phase of the disease through serous fluid from the blisters during sexual intercourse, and via breeding tools, handlers, etc.

Clinical signs include cute small lesions, no bigger than 2 mm in diameter around the vulva in mares, and on the sheath in stallions. The small bumps blister and then rupture, leaving raw, ulcerated, painful sores. While the majority of the symptoms are external, the presence of the virus can cause small and large plaque variants in tissues.

Eating raw or semi-cooked infected liver or lymph nodes infected with nymphal L. serrata causes severe symptoms in the human nasopharynx. Submaxillary and cervical lymph nodes sometimes enlarge and the neck is swollen. Complications include abscesses in the auditory canals, facial paralysis, and enlarged tonsils producing asphyxiation. These symptoms are well recognized as a disease call “halzoun syndrome” in Lebanon and nearby countries.

In Egypt, infected camels and buffalo may also be a source of infection for dogs, which are companions of man in desert and semi-desert areas where grazing is a major profession, and in villages, where dogs are also common. Infected dogs, in turn, are a source of infection to man who may be an intermediate host.

While the prognosis of cryofibrinoginemic disease varies greatly depending on its severity as well as the severity of its associated disorders, satisfactory clinical outcomes are reported in 50-80% of patients with primary or secondary disease treated with corticosteroid and/or immunosuppressive regimens. However, relapses occur within the first 6 months after stopping or decreasing therapy in 40-76% of cases. Sepsis resulting from infection of necrotic tissue is the most common threat to life in primary disease whereas the associated disorder is a critical determinant of prognosis in secondary disease.

There is antibiotic therapy for secondary infections caused by the parasite. However, surgical removal is usually the only way to get rid of the parasites.

The Jarisch-Herxheimer reaction, which is the response to the body after endotoxins are released by the death of harmful organisms in the human body, starts usually during the first day of antibiotic treatment. The reaction increases the person's body temperature, decreases the overall blood pressure (both systolic and diastolic levels), and results in leukopenia and rigors in the body. This reaction can occur during any treatment of spirochete diseases.

It is important to realize that syphilis can recur. An individual who has had the disease once, even if it has been treated, does not prevent the person from experiencing recurrence of syphilis. Individuals can be re-infected, and because syphilis sores can be hidden, it may not be obvious that the individual is infected with syphilis. In these cases, it is vital to become tested and treated immediately to reduce spread of the infection.

Rates of infection increase in conditions of crowding and poor sanitation, and are higher in military personnel and mental institutions.

The true extent of disease has yet to emerge, as most laboratories do not use techniques to adequately identify this organism. An Australian study identified a large number of patients, considered to have irritable bowel syndrome, who were actually infected with "Dientamoeba fragilis".

Although "D. fragilis" has been described as an infection "emerging from obscurity", it has become one of the most prevalent gastrointestinal infections in industrialized countries, especially among children and young adults. A Canadian study reported a prevalence of around 10% in boys and girls aged 11–15 years, a prevalence of 11.5% in individuals aged 16–20, and a lower incidence of 0.3–1.9% in individuals over age 20.

Success in treating the primary disease has been reported using blood clot lysing agents such as anabolic steroids (e.g. danazol or stanozolol which is no longer available in the USA), streptokinase, and streptodornase; anticoagulants such as heparin and warfarin, and immunosuppressive drug regimens such as a corticosteroid (e.g. prednisone) combined with either azathioprine of chlorambucil. Very moderate cases may do well by simply avoiding cold exposure. Treatment with a corticosteroid plus low-dose aspirin followed by maintenance therapy with an anabolic steroid where necessary are recommended for moderately severe cases. Very severe cases generally require an immunosuppressive drug regimen and if extreme or life threatening require resorting to plasmaphoresis or plasma exchange. Cryofiltration apheresis, a method to remove plasma agents by removing cold-induced precipitated material, may be an effective alternative to plasmaphoresis and plasma exchange but is still regarded as second-line therapy for cryofibirnogenemic disease treatment.

During the several years following its initial diagnosis, some 27-47% of primary cryofibrinoginemic diseases are complicated by the development of a B-cell or T-cell lymphoma. That is, the cryofibrinoginemic disease may appear to precede by years the malignant disorder to which it is associated. Accordingly, patients require careful follow-up not only to treat their primary cryofibrinoginemic disease but also to monitor them for movement to the diagnosis of secondary cryofibrinoginemic disease caused by the development of one of these hematological malignancies.

As many individuals are asymptomatic carriers of "D. fragilis", pathogenic and nonpathogenic variants are proposed to exist. A study of "D. fragilis" isolates from 60 individuals with symptomatic infection in Sydney, Australia, found all were infected with the same genotype, which is the most common worldwide, but differed from the genotype first described from a North American isolate and later also detected in Europe.

Pacheco's disease is an eponymously named disease; it is named after the Brazilian veterinarian, Genesio Pacheco, who first came across the disease in 1929, in an outbreak affecting the turquoise-fronted amazon parrot, "Amazona aestiva". Initially, Pacheco's disease was thought to be a manifestation of avian psittacosis. The causative agent of the disease, a herpesvirus, was not identified until 1975.

All patients with symptomatic cryoglobulinemia are advised to avoid, or protect their extremities, from exposure to cold temperatures. Refrigerators, freezers, and air-conditioning represent dangers of such exposure.

Individuals found to have circulating cryoglobulins but no signs or symptoms of cryoglobulinemic diseases should be evaluated for the possibility that their cryoglobulinemia is a transient response to a recent or resolving infection. Those with a history of recent infection that also have a spontaneous and full resolution of their cryoglobulinemia need no further treatment. Individuals without a history of infection and not showing resolution of their cryoglobulinemia need to be further evaluated. Their cryoglobulins should be analyzed for their composition of immunoglobulin type(s) and complement component(s) and examined for the presence of the premalignant and malignant diseases associated with Type I disease as well as the infectious and autoimmune diseases associated with type II and type III disease. A study conducted in Italy on >140 asymptomatic individuals found five cases of hepatitis C-related and one case of hepatitis b-related cryoglobulinemia indicating that a complete clinical examination of asymptomatic individuals with cryoglobulinemia offers a means for finding people with serious but potentially treatable and even curable diseases. Individuals who show no evidence of a disease underlying their cryoglobulinemia and who remain asymptomatic should be followed closely for any changes that may indicate development of cryoglobulinemic disease.

Lhermitte–Duclos disease is a rare entity; approximately 222 cases of LDD have been reported in medical literature. Symptoms of the disease most commonly manifest in the third and fourth decades of life, although it may onset at any age. Men and women are equally affected, and there is not any apparent geographical pattern.

Jessner lymphocytic infiltrate of the skin is a cutaneous condition characterized by a persistent papular and plaque-like skin eruption which can occur on the neck, face and back and may re-occur. This is an uncommon skin disease and is a benign collection of lymph cells. Its cause is not known and can be hereditary. It is named for Max Jessner. It is thought to be equivalent to lupus erythematosus tumidus.

It can occur as the result of ACE inhibitors and a number of medications used to treat multiple sclerosis including glatiramer acetate.

Specific age groups, persons who participate in risky sexual behavior, or those have certain health conditions may require screening. The CDC recommends that sexually active women under the age of 25 and those over 25 at risk should be screened for chlamydia and gonorrhea yearly. Appropriate times for screening are during regular pelvic examinations and preconception evaluations. Nucleic acid amplification tests are the recommended method of diagnosis for gonorrhea and chlamydia. This can be done on either urine in both men and women, vaginal or cervical swabs in women, or urethral swabs in men. Screening can be performed:

- to assess the presence of infection and prevent tubal infertility in women

- during the initial evaluation before infertility treatment

- to identify HIV infection

- for men who have sex with men

- for those who may have been exposed to hepatitis C

- for HCV