"Acanthamoeba spp." causes mostly subacute or chronic granulomatous amoebic encephalitis (GAE), with a clinical picture of headaches, altered mental status, and focal neurologic deficit, which progresses over several weeks to death. In addition, "Acanthamoeba spp." can cause granulomatous skin lesions and, more seriously, keratitis and corneal ulcers following corneal trauma or in association with contact lenses.

Free-living amoebae (or "FLA") in the Amoebozoa group are important causes of disease in humans and animals.

"Naegleria fowleri" is sometimes included in the group "free-living amoebae", and it causes a condition traditionally called primary amoebic meningoencephalitis. However, Naegleria is now considered part of the Excavata, not the Amoebozoa, and is considered to be much more closely related to "Leishmania" and "Trypanosoma".

Onset of symptoms begins one to nine days following exposure (with an average of five). Initial symptoms include changes in taste and smell, headache, fever, nausea, vomiting, back pain, and a stiff neck. Secondary symptoms are also meningitis-like including confusion, hallucinations, lack of attention, ataxia, cramp and seizures. After the start of symptoms, the disease progresses rapidly over three to seven days, with death usually occurring anywhere from seven to fourteen days later, although it can take longer. In 2013, a man in Taiwan died twenty-five days after being infected by "Naegleria fowleri".

It affects healthy children or young adults who have recently been exposed to bodies of fresh water. Some people have presented with a clinical triad of edematous brain lesions, immune suppression, and fever.

Naegleriasis (also known as primary amoebic meningoencephalitis) is an infection of the brain by the free-living unicellular "Naegleria fowleri".

"N. fowleri" is typically found in warm bodies of fresh water, such as ponds, lakes, rivers, and hot springs. It is also found in soil, poorly maintained municipal water supplies, water heaters, near warm-water discharges of industrial plants, and in poorly chlorinated or unchlorinated swimming pools, in an amoeboid or temporary flagellate stage. There is no evidence of it living in salt water. As the disease is rare, it is often not considered. Symptoms are similar to those of meningitis.

Although infection occurs rarely, it nearly always results in death, with a case fatality rate greater than 95%.

Microsporidiosis is an opportunistic intestinal infection that causes diarrhea and wasting in immunocompromised individuals (HIV, for example). It results from different species of microsporidia, a group of microbial (unicellular) fungi.

In HIV infected individuals, microsporidiosis generally occurs when CD4+ T cell counts fall below 150.

Although it is classified as a protozoal disease in ICD-10, their phylogenetic placement has been resolved to be within the Fungi, and some sources classify microsporidiosis as a mycosis, however, they are highly divergent and rapidly evolving.

Human’s clinical signs consisted of swelling and eye infections. There were nodules underneath the skin, abscesses or cysts, and lesions running throughout the body. There were papules, plaques and granulomatous damages on the body. In extreme cases there were deep infections within the eyes, bones, heart and central nervous system.

In general, the term "infestation" refers to parasitic diseases caused by animals such as arthropods (i.e. mites, ticks, and lice) and worms, but excluding conditions caused by protozoa, fungi, bacteria, and viruses, which are called infections.

Granulomatous amoebic encephalitis (GAE) is a central nervous system disease caused by certain species of free-living amoebae, especially species of "Acanthamoeba" and "Balamuthia mandrillaris".

The term is most commonly used with "Acanthamoeba". In more modern references, the term "balamuthia amoebic encephalitis" (BAE) is commonly used when "Balamuthia mandrillaris" is the cause.

GAE may present in numerous ways. There is no solid definition, as only a handful of patients have presented thus far with GAE. GAE can present with: focal paralysis, seizures, brainstem symptoms, and other neurological problems, some of which may mimic glioma (especially brainstem glioma), or other brain diseases, which may hamper timely diagnosis. These symptoms are caused by inflammatory necrosis of brain tissue brought on by amoebic infiltrates.

Protozoan infections are parasitic diseases caused by organisms formerly classified in the Kingdom Protozoa. They include organisms classified in Amoebozoa, Excavata, and Chromalveolata.

Examples include "Entamoeba histolytica", "Plasmodium" (some of which cause malaria), and "Giardia lamblia". "Trypanosoma brucei", transmitted by the tsetse fly and the cause of African sleeping sickness, is another example.

The species traditionally collectively termed "protozoa" are not closely related to each other, and have only superficial similarities (eukaryotic, unicellular, motile, though with exceptions). The terms "protozoa" (and protist) are usually discouraged in the modern biosciences. However, this terminology is still encountered in medicine. This is partially because of the conservative character of medical classification, and partially due to the necessity of making identifications of organisms based upon appearances and not upon DNA.

Protozoan infections in animals may be caused by organisms in the sub-class Coccidia (disease: Coccidiosis) and species in the genus "Besnoitia" (disease: Besnoitiosis).

Several pathogenic protozoans appear to be capable of sexual processes involving meiosis (or at least a modified form of meiosis). Included among these protozoans are "Plasmodium falciparum" (malaria), "Toxoplasma gondii" (toxoplasmosis), "Leishmania" species (leishmaniases), "Trypanosoma brucei" (African sleeping sickness), "Trypanosoma cruzi" (Chagas disease) and "Giardia intestinalis" (giardiasis).

Infestations can be classified as either external or internal with regards to the parasites' location in relation to the host.

External or ectoparasitic infestation is a condition in which organisms live primarily on the surface of the host (though porocephaliasis can penetrate viscerally) and includes those involving mites, ticks, head lice and bed bugs.

An internal (or endoparasitic) infestation is a condition in which organisms live within the host and includes those involving worms (though swimmer's itch stays near the surface).

Medically, the term "infestation" is often reserved only for external ectoparasitic infestations while the term "infection" refers to internal endoparasitic conditions.

From birds to equines, Phaeohyphomycosis persists and has a massive range of clinical signs throughout differing species. Poultry and wild birds had neurological disorders and a loss of movement control. They experienced severe torticollis, which are severe muscle spasms that compromise the bird’s ability to hold up its head. The birds exhibited a loss of balance due to the rigidity of their legs.

Cats showed signs of difficulty breathing due to excessive swelling of the nose. There were also lesions found throughout the body, including the brain. In extreme cases dogs exhibited vision impairment and had deep infections in the nasal cavity, kidneys, and the cerebellum. In dogs, brain infections were found similar to infections that were found in humans. Other clinical signs were lesions, abscesses, and severe inflammation throughout the dog’s body.

Ruminants and equines were affected the same way from Phaeohyphomycosis. They showed respiratory distress through constant coughing and a fever. They demonstrated signs of anorexia, lethargy, and hypothermia. There was inflammation, hair loss, scaling, and damage to their cerebellum.

"Balamuthia" infection is a cutaneous condition resulting from "Balamuthia" that may result in various skin lesions.

"Balamuthia mandrillarisis" a free-living amoeba (a single-celled living organism) found in the environment. It is one of the causes of granulomatous amoebic encephalitis (GAE), a serious infection of the brain and spinal cord. "Balamuthia" is thought to enter the body when soil containing it comes in contact with skin wounds and cuts, or when dust containing it is breathed in or gets in the mouth. The "Balamuthia" amoebae can then travel to the brain through the blood stream and cause GAE. GAE is a very rare disease that is usually fatal.

Scientists at the Centers for Disease Control and Prevention (CDC) first discovered "Balamuthia mandrillaris" in 1986. The amoeba was found in the brain of a dead mandrill. After extensive research, "B. mandrillaris" was declared a new species in 1993. Since then, more than 200 cases of "Balamuthia" infection have been diagnosed worldwide, with at least 70 cases reported in the United States. Little is known at this time about how a person becomes infected.

Flacherie (literally: "flaccidness") is a disease of silkworms, caused by silkworms eating infected or contaminated mulberry leaves. Flacherie infected silkworms look weak and can die from this disease. Silkworm larvae that are about to die from Flacherie are a dark brown.

There are two kinds of flacherie: essentially, infectious (viral) flacherie and noninfectious ("bouffee") flacherie. Both are technically a lethal diarrhea.

Bouffée flacherie is caused by heat waves ("bouffée" means "sudden heat spell" in French).

Viral flacherie is ultimately caused by infection with "Bombyx mori" infectious flacherie virus (BmIFV, Iflaviridae), "Bombyx mori" densovirus (BmDNV, Parvoviridae) or "Bombyx mori" cypovirus 1 (BmCPV-1, Reoviridae). This either alone or in combination with bacterial infection destroys the gut tissue. Bacterial pathogens contributing to infectious flaccherie are "Serratia marcescens", and species of "Streptococcus" and "Staphylococcus" in the form known as thatte roga.

Louis Pasteur, who began his studies on silkworm diseases in 1865, was the first one able to recognize that mortality due to viral flacherie was caused by infection. (Priority, however, was claimed by Antoine Béchamp.) Richard Gordon described the discovery: "The French silk industry was meanwhile plummeting from a 130 million to an 8 million francs annual income, because the silkworms had all caught "pébrine," black pepper disease…He [Pasteur] went south from Paris to Alais, and rewarded them by discovering the silkworm epidemic to be inflicted by some sort of living microbe…Pasteur threw in another disease, "flâcherie," silkworm diarrhoea. The cures for both were culling the insects which showed the peppery spots — the peasants bottled the silkworm moths in brandy, for display to the experts — and rigorous hygiene of the mulberry leaf."

Symptoms are different for every person depending on the type of rat-bite fever with which the person is infected. Both spirillary and streptobacillary rat-bite fever have a few individual symptoms, although most symptoms are shared. Streptobacillosis is most commonly found in the United States and spirillary rat-bite fever is generally diagnosed in patients in Africa. Rat-bite symptoms are visually seen in most cases and include inflammation around the open sore. A rash can also spread around the area and appear red or purple. Other symptoms associated with streptobacillary rat-bite fever include chills, fever, vomiting, headaches, and muscle aches. Joints can also become painfully swollen and pain can be experienced in the back. Skin irritations such as ulcers or inflammation can develop on the hands and feet. Wounds heal slowly, so symptoms possibly come and go over the course of a few months.

Symptoms associated with spirillary rat-bite fever include issues with the lymph nodes, which often swell or become inflamed as a reaction to the infection. The most common locations of lymph node swelling are in the neck, groin, and underarm. Symptoms generally appear within 2 to 10 days of exposure to the infected animal. It begins with the fever and progresses to the rash on the hands and feet within 2 to 4 days. Rash appears all over the body with this form, but rarely causes joint pain.

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.

Although organisms such as bacteria function as parasites, the usage of the term "parasitic disease" is usually more restricted. The three main types of organisms causing these conditions are protozoa (causing protozoan infection), helminths (helminthiasis), and ectoparasites. Protozoa and helminths are usually endoparasites (usually living inside the body of the host), while ectoparasites usually live on the surface of the host. Occasionally the definition of "parasitic disease" is restricted to diseases due to endoparasites.

The streptobacillosis form of rat-bite fever is known by the alternative names Haverhill fever and epidemic arthritic erythema. It is a severe disease caused by "Streptobacillus moniliformis", transmitted either by rat bite or ingestion of contaminated products (Haverhill fever). After an incubation period of 2–10 days, Haverhill fever begins with high prostrating fevers, rigors (shivering), headache, and polyarthralgia (joint pain). Soon, an exanthem (widespread rash) appears, either maculopapular (flat red with bumps) or petechial (red or purple spots) and arthritis of large joints can be seen. The organism can be cultivated in blood or articular fluid. The disease can be fatal if untreated in 20% of cases due to malignant endocarditis, meningoencephalitis, or septic shock. Treatment is with penicillin, tetracycline, or doxycycline.

Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens.

Specific defences are specialised responses to particular pathogens recognised by the fish's body, that is adaptative immune responses. In recent years, vaccines have become widely used in aquaculture and ornamental fish, for example vaccines for furunculosis in farmed salmon and koi herpes virus in koi.

Some commercially important fish diseases are VHS, ich and whirling disease.

Head lice are generally uncomfortable, but typically do not constitute a serious condition. The most common symptom is itching of the head, which normally worsens 3 to 4 weeks after the initial infestation. The bite reaction is very mild, and it can be rarely seen between the hairs. Bites can be seen, especially in the neck of long-haired individuals when the hair is pushed aside. Swelling of the local lymph nodes and fever are rare. Itching may cause skin breakdown and uncommonly result in a bacterial infection.

In Ethiopia, head lice appear to be able to spread louse-born epidemic typhus and "Bartonella quintana". In Europe, the head lice do not appear to carry these infections.

A lethal infection in mink, the Aleutian disease virus lies dormant in ferrets until stress or injury allows it to surface. While the parvovirus itself causes little or no harm to the ferret host, the large number of antibodies produced in response to the presence of the virus results in a systemic vasculitis, resulting in eventual renal failure, bone marrow suppression and death.

The symptoms are chronic, progressive weight loss, lethargy, splenomegaly (enlarged spleen), anemia, rear leg weakness, seizures and black tarry stool. Additional symptoms include poor reproduction and/or oral bleeding/gastrointestinal bleeding. Lesions can also be found within the pelt depending on the severity of the disease. This virus can unfortunately reduce fitness of wild mink especially, by disturbing both the productivity within adult females and the overall survivor rates of both juveniles and adults. Likewise, in the mink kits that survive, it infects the alveolar cells and ultimately causes respiratory distress, possibly leading to death.

Once symptoms show themselves, the disease progresses rapidly, usually to death within a few months.

A parasitic disease, also known as parasitosis, is an infectious disease caused or transmitted by a parasite. Many parasites do not cause diseases. Parasitic diseases can affect practically all living organisms, including plants and mammals. The study of parasitic diseases is called parasitology.

Some parasites like "Toxoplasma gondii" and "Plasmodium" spp. can cause disease directly, but other organisms can cause disease by the toxins that they produce.

Lymphadenitis, the swelling of the lymph nodes, is a commonly recognized symptom of many diseases. An early manifestation of filariasis, lymphadenitis more frequently occurs in the inguinal area during "B. malayi" infection and can occur before the worms mature.

The first potential reaction is an itchy, papular rash that results from cercariae penetrating the skin, often in a person's first infection. The round bumps are usually one to three centimeters across. Because people living in affected areas have often been repeatedly exposed, acute reactions are more common in tourists and migrants. The rash can occur between the first few hours and a week after exposure and lasts for several days. A similar, more severe reaction called "swimmer's itch" reaction can also be caused by cercariae from animal trematodes that often infect birds.