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.

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.

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.

Cryptosporidiosis is a parasitic disease that is transmitted through contaminated food or water from an infected person or animal. Cryptosporidiosis in cats is rare, but they can carry the protozoan without showing any signs of illness. Cryptosporidiosis can cause profuse, watery diarrhea with cramping, abdominal pain, and nausea in people. Illness in people is usually self-limiting and lasts only 2–4 days, but can become severe in people with weakened immune systems. Cryptosporidiosis (Cryptosporidium spp.) Cats transmit the protozoan through their feces. The symptoms in people weight loss and chronic diarrhea in high-risk patients. More than one species of this genus can be acquired by people. Dogs can also transmit this parasite.

Most infected cats have been healthy before a very sudden onset of severe disease. The course of clinical disease is often swift with clinical signs of lethargy and inappetence within 5 to 20 days after the tick bite. Cats develop a high fever, but the temperature may become low before death. Other clinical findings can be: dehydration, icterus (jaundice), enlarged liver and spleen, lymphadenopathy, pale mucus membranes, respiratory distress, tachycardia or bradycardia, and tick infestation (although ticks are not often found on infected cats since cats typically groom ticks off their fur). Signs of disease seen on blood work include hemolytic anemia, thrombocytopenia, increased or decreased white blood cell numbers, icterus, and elevated liver enzymes. Death usually follows the onset of clinical signs within a few days. However, more recent studies show not all cats develop clinical signs after infection, and some cats survive the infection.

Physiological reactions to "Toxocara" infection depend on the host’s immune response and the parasitic load. Most cases of "Toxocara" infection are asymptomatic, especially in adults. When symptoms do occur, they are the result of migration of second stage "Toxocara" larvae through the body.

Covert toxocariasis is the least serious of the three syndromes and is believed to be due to chronic exposure. Signs and symptoms of covert toxocariasis are coughing, fever, abdominal pain, headaches, and changes in behavior and ability to sleep. Upon medical examination, wheezing, hepatomegaly, and lymphadenitis are often noted.

High parasitic loads or repeated infection can lead to visceral larva migrans (VLM). VLM is primarily diagnosed in young children, because they are more prone to exposure and ingestion of infective eggs. "Toxocara" infection commonly resolves itself within weeks, but chronic eosinophilia may result. In VLM, larvae migration incites inflammation of internal organs and sometimes the central nervous system. Symptoms depend on the organ(s) affected. Patients can present with pallor, fatigue, weight loss, anorexia, fever, headache, rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. Sometimes the subcutaneous migration tracks of the larvae can be seen. Patients are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (IgM, IgG, and IgE classes), leucocytosis, and elevated anti-A and –B isohaemagglutinins. Severe cases have occurred in people who are hypersensitive to allergens; in rare cases, epilepsy, inflammation of the heart, pleural effusion, respiratory failure, and death have resulted from VLM.

Ocular larva migrans (OLM) is rare compared with VLM. A light "Toxocara" burden is thought to induce a low immune response, allowing a larva to enter the host’s eye. Although there have been cases of concurrent OLM and VLM, these are extremely exceptional. OLM often occurs in just one eye and from a single larva migrating into and encysting within the orbit. Loss of vision occurs over days or weeks. Other signs and symptoms are red eye, white pupil, fixed pupil, retinal fibrosis, retinal detachment, inflammation of the eye tissues, retinal granulomas, and strabismus. Ocular granulomas resulting from OLM are frequently misdiagnosed as retinoblastomas. "Toxocara" damage in the eye is permanent and can result in blindness.

A case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with "Toxocara". In this study, the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive ELISA for "T. cani"s.

The most common symptom is coughing and other typical symptoms are wheezing and weight loss. These symptoms are caused by larvae that reside in the lungs where immunity develops and the accumulation of mucus cause blockage of the airway into the lungs.

Cytauxzoon felis is a protozoal organism transmitted to domestic cats by tick bites, and whose natural reservoir host is the bobcat. "C. felis" has been found in other wild felid species such as Florida bobcat, eastern bobcat, Texas cougar, and a white tiger in captivity. "C. felis" infection is limited to the family felidae which means that "C. felis" poses no zoonotic (transmission to humans) risk or agricultural (transmission to farm animals) risk. Until recently it was believed that after infection with "C. felis", pet cats almost always died. As awareness of "C. felis" has increased it has been found that treatment is not always futile. More cats have been shown to survive the infection than was previously thought. New treatments offer as much as 60% survival rate.

The several forms of the infection are:

- Skin/subcutaneous tissue disease is a septic phlegmon that develops classically in the hand and forearm after a cat bite. Inflammatory signs are very rapid to develop; in 1 or 2 hours, edema, severe pain, and serosanguineous exudate appear. Fever, moderate or very high, can be seen, along with vomiting, headache, and diarrhea. Lymphangitis is common. Complications are possible, in the form of septic arthritis, osteitis, or evolution to chronicity.

- Sepsis is very rare, but can be as fulminant as septicaemic plague, with high fever, rigors, and vomiting, followed by shock and coagulopathy.

- Pneumonia disease is also rare and appears in patients with some chronic pulmonary pathology. It usually presents as bilateral consolidating pneumonia, sometimes very severe.

- Zoonosis, pasteurellosis can be transmitted to humans through cats.

Other locations are possible, such as septic arthritis, meningitis, and acute endocarditis, but are very rare.

Diagnosis is made with isolation of "Pasteurella multocida" in a normally sterile site (blood, pus, or cerebrospinal fluid).

If an animal is suspected of lungworm infection, there are many ways to detect this parasitic infection such as performing one or more of the following techniques: a complete medical history including lung auscultation (stethoscope examination), doing a chest xray, fecal examination for detection of ova or larvae, examination of respiratory secretions for ova or larvae, and/or a complete blood count (CBC) to check for signs of increase in eosinophils

The incubation period for "Toxocara canis" and "cati" eggs depends on temperature and humidity. "T. canis" females, specifically, are capable of producing up to 200,000 eggs a day that require 2-6 weeks minimum up to a couple months before full development into the infectious stage. Under ideal summer conditions, eggs can mature to the infective stage after two weeks outside of a host. Provided sufficient oxygen and moisture availability, "Toxocara" eggs can remain infectious for years, as their resistant outer shell enables the protection from most environmental threats.However, as identified in a case study presented within the journal of helminthology, the second stage of larvae development poses strict vulnerabilities to certain environmental elements. High temperatures and low moisture levels will quickly degrade the larvae during this stage of growth.

Dependent on the infectious syndrome, symptoms include fever, fatigue, dry cough, headache, blurred vision, and confusion. Symptom onset is often subacute, progressively worsened over several weeks. The two most common presentations are meningitis (an infection in and around the brain) and pulmonary (lung) infection.

Detection of cryptococcal antigen (capsular material) by culture of CSF, sputum and urine provides definitive diagnosis. Blood cultures may be positive in heavy infections. India ink of the CSF is a traditional microscopic method of diagnosis, although the sensitivity is poor in early infection, and may miss 15-20% of patients with culture-positive cryptococcal meningitis. Unusual morphological forms are rarely seen. Cryptococcal antigen from cerebrospinal fluid is the best test for diagnosis of cryptococcal meningitis in terms of sensitivity. Apart from conventional methods of detection like direct microscopy and culture, rapid diagnostic methods to detect cryptococcal antigen by latex agglutination test, lateral flow immunochromatographic assay (LFA), or enzyme immunoassay (EIA). A new cryptococcal antigen LFA was FDA approved in July 2011. Polymerase chain reaction (PCR) has been used on tissue specimens.

Cryptococcosis can rarely occur in the non-immunosuppressed people, particularly with "Cryptococcus gattii".

Cryptococcosis, also known as cryptococcal disease, is a potentially fatal fungal disease. It is caused by one of two species; "Cryptococcus neoformans" and "Cryptococcus gattii". These were all previously thought to be subspecies of "C. neoformans" but have now been identified as distinct species.

Cryptococcosis is believed to be acquired by inhalation of the infectious propagule from the environment. Although the exact nature of the infectious propagule is unknown, the leading hypothesis is the basidiospore created through sexual or asexual reproduction.

Cat bites are usually considered as minor injuries but can result in serious infection. Not all infections that can be obtained from exposure to a cat are transmitted through a cat bite, like plague.

The rabbit ear mite, "Psoroptes cuniculi", is larger than "Otodectes cynotis". It causes thick firm debris to form in the ear canal, and can eventually migrate to the skin of the outer ear and face. Symptoms include scratching and shaking of the head. Treatment includes topical selamectin, or injections of ivermectin and frequent cleanings of the rabbit's environment.

The hallmark clinical sign of effusive FIP is the accumulation of fluid within the abdomen or chest, which can cause breathing difficulties. Other symptoms include lack of appetite, fever, weight loss, jaundice, and diarrhea.

Dry FIP will also present with lack of appetite, fever, jaundice, diarrhea, and weight loss, but there will not be an accumulation of fluid. Typically a cat with dry FIP will show ocular or neurological signs. For example, the cat may develop difficulty in standing up or walking, becoming functionally paralyzed over time. Loss of vision is another possible outcome of the disease.

The diagnosis is aided by obtaining a history of the circumstances surrounding the bite. The time the bite was experienced, the location of the bite, and examination of the bite is noted. The person may have drainage from the site of the bite. They may also be febrile. Swelling may also occur. Because the wound from the bite may have healed over the punctures, the wound it may be opened and explored. The site is anesthetized prior to exploration of the wound for is examined for damage. Neurovascular status is assessed. Immune status may determine treatment as does

the presence of transplanted tissue or organs, rheumatic disease, diabetes, HIV/AIDS and sickle cell disease.

Swollen glands (lymph nodes) and red streaks radiating upward may be evident.

The diagnosis of a cat with rabies is evident by observing the cat. Cats with rabies may also appear restless, pant, and attack other animals, people, or objects. Animals with rabies typically die within a few days of appearing sick. Vaccination of the cat can prevent rabies being transmitted by the cat through a bite. If the cat is suspected of being infected with rabies, the person begins treatment with rabies vaccine.

The ear mite is the most common cause of ear infections in cats, quickly spreading from one cat to another through direct contact. Ear mites cause inflammatory symptoms, similar to bacterial and yeast infections. Symptoms include itching and redness of the ears. Other, more serious problems can result from untreated infections, such as skin disease in areas other than the ear like the neck and tail, and deafness.

Cat-scratch disease commonly presents as tender, swollen lymph nodes near the site of the inoculating bite or scratch or on the neck, and is usually limited to one side. This condition is referred to as regional lymphadenopathy and occurs 1–3 weeks after inoculation. Lymphadenopathy in CSD most commonly occurs in the arms, neck, or jaw, but may also occur near the groin or around the ear. A vesicle or an erythematous papule may form at the site of initial infection. Most patients also develop systemic symptoms such as malaise, decreased appetite, and aches. Other associated complaints include headache, chills, muscular pains, joint pains, arthritis, backache, and abdominal pain. It may take 7 to 14 days, or as long as two months, for symptoms to appear. Most cases are benign and self-limiting, but lymphadenopathy may persist for several months after other symptoms disappear. The disease usually resolves spontaneously, with or without treatment, in one month.

In rare situations, CSD can lead to the development of serious neurologic or cardiac sequelae such as meningoencephalitis, encephalopathy, seizures, or endocarditis. Endocarditis associated with "Bartonella" infection has a particularly high mortality. Parinaud's oculoglandular syndrome is the most common ocular manifestation of CSD, and is a granulomatous conjunctivitis with concurrent swelling of the lymph node near the ear. Optic neuritis or neuroretinitis is one of the atypical presentations.

Immunocompromised patients are susceptible to other conditions associated with "B. henselae" and "B. quintana", such as bacillary angiomatosis or bacillary peliosis. Bacillary angiomatosis is primarily a vascular skin lesion that may extend to bone or be present in other areas of the body. In the typical scenario, the patient has HIV or another cause of severe immune dysfunction. Bacillary peliosis is caused by "B. henselae" that most often affects patients with HIV and other conditions causing severe immune compromise. The liver and spleen are primarily affected, with findings of blood-filled cystic spaces on pathology. In 2015 a Toledo, Ohio woman lost eyesight in an eye after a cat licked it.

Cat-scratch disease (CSD) is a common and usually benign infectious disease caused by the bacterium "Bartonella henselae". It is most commonly found in children following a scratch or bite from a cat within about one to two weeks.

"B. henselae" is the etiologic agent for peliosis hepatis, which is defined as a vascular proliferation of sinusoid hepatic capillaries resulting in blood-filled spaces in the liver in HIV patients and organ transplant recipients. Peliosis hepatis can be associated with peliosis of the spleen, as well as bacillary angiomatosis of the skin in HIV patients.

Patients can develop two clinical phases: an acute septic phase and a chronic eruptive phase associated with skin lesions. In the acute phase (also known as Oroya fever or "fiebre de la Oroya"), "B. bacilliformis" infection is a sudden, potentially life-threatening infection associated with high fever and decreased levels of circulating red blood cells (i.e., hemolytic anemia)and transient immunosuppression. "B. bacilliformis" is considered the most deadly species to date, with a death rate of up to 90% during the acute phase, which typically lasts two to four weeks. Peripheral blood smears show anisomacrocytosis with many bacilli adherent to red blood cells. Thrombocytopenia is also seen and can be very severe. Neurologic manifestations (neurobartonellosis) are altered mental status, agitation, or even coma, ataxia, spinal meningitis, or paralysis. It is seen in 20% of patients with acute infection, in which the prognosis is very guarded with an about 50% mortality. The most feared complication is overwhelming infection mainly by Enterobacteriaceae, particularly "Salmonella" (both "S. typhi" and " S. "non-"typhi", as well as reactivation of toxoplasmosis and other opportunistic infections .

The chronic manifestation consists of a benign skin eruption with raised, reddish-purple nodules (angiomatous tumours). The bacterium can be seen microscopically, if a skin biopsy is silver stained (the Warthin–Starry method).

Toxoplasma chorioretinitis, more simply known as ocular toxoplasmosis, is probably the most common cause of infections in the back of the eye (posterior segment) worldwide. The causitive agent is "Toxoplasma gondii", and in the United States, most cases are acquired congenitally. The most common symptom is decreased visual acuity in one eye. The diagnosis is made by examination of the eye, using ophthalmoscopy. Sometimes serologic testing is used to rule out the disease, but due to high rates of false positives, serologies are not diagnostic of toxoplasmic retinitis.

If vision is not compromised, treatment may not be necessary. When vision is affected or threatened, treatment consists of pyrimethamine, sulfadiazine, and folinic acid for 4–6 weeks. Prednisone is sometimes used to decrease inflammation.