Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation. However, many were present in northern Europe and northern America in the 17th and 18th centuries before modern understanding of disease causation. The initial impetus for tropical medicine was to protect the health of colonialists, notably in India under the British Raj. Insects such as mosquitoes and flies are by far the most common disease carrier, or vector. These insects may carry a parasite, bacterium or virus that is infectious to humans and animals. Most often disease is transmitted by an insect "bite", which causes transmission of the infectious agent through subcutaneous blood exchange. Vaccines are not available for most of the diseases listed here, and many do not have cures.

Human exploration of tropical rainforests, deforestation, rising immigration and increased international air travel and other tourism to tropical regions has led to an increased incidence of such diseases.

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.

Infections by "Mansonella perstans", while often asymptomatic, can be associated with angioedema, pruritus, fever, headaches, arthralgias, and neurologic manifestations. "Mansonella streptocerca" can manifest on the skin via pruritus, papular eruptions and pigmentation changes. "Mansonella ozzardi" can cause symptoms that include arthralgias, headaches, fever, pulmonary symptoms, adenopathy, hepatomegaly, and pruritus. Eosinophilia is often prominent in all cases of Mansonelliasis. "M. perstans" can also present with Calabar-like swellings, hives, and a condition known as Kampala, or Ugandan eye worm. This occurs when adult M. perstans invades the conjunctiva or periorbital connective tissues in the eye. "M. perstans" can also present with hydrocele in South America. However, it is often hard to distinguish between the symptoms of Mansonelliasis and other nematode infections endemic to the same areas.

Lymphangitis is the inflammation of the lymphatic vessels in response to infection. It occurs early in the course of infection in response to worm development, molting, death, or bacterial and fungal infection. The affected lymphatic vessel becomes distended and tender, and the overlying skin becomes erythemous and hot. Abscess formation and ulceration of the affected lymph node occasionally occurs during "B. malayi infection", more readily than in Bancroftian filariasis. Remnants of adult worms can sometimes be found in the ulcer drainage.

Acanthocheilonemiasis is a rare tropical infectious disease caused by a parasite known as "Acanthocheilonema perstans". It can cause skin rashes, abdominal and chest pains, muscle and joint pains, neurological disorders and skin lumps. It is mainly found in Africa. The parasite is transmitted through the bite of small flies. Studies show that there are elevated levels of white blood cells.

Acanthocheilonemiasis belongs to a group of parasitic diseases known as filarial disease (nematode), all of which are classified as Neglected Tropical Diseases. Filarial disease results when microfilariae, which are nematode larvae, reach the lymphatic system; microfilariae reside in the serous cavities of humans. They have a five-stage life cycle that includes birth to thousands of live microfilariae within the host (i.e. human body), and then translocation via blood meal to the dermis layer of the skin. It is here that microfilariae cause major symptoms, which are edema and thickening of the skin and underlying connective tissues. It can also cause skin rashes, abdominal and chest pains, muscle (myalgia) and joint pains, neurological disorders and skin lumps. In addition, it causes spleen and liver enlargement, which is called hepatosplenomegaly. Studies show elevated levels of leukocytes, or white blood cells, which is referred to as eosinophilia. It is mainly found in Africa. The parasite is transmitted through the bite of small flies ("A. coliroides").

The most spectacular symptom of lymphatic filariasis is elephantiasis, a stage 3 lymphedema with thickening of the skin and underlying tissues. This was the first mosquito-borne disease to be discovered. Elephantiasis results when the parasites lodge in the lymphatic system and cause blockages to the flow of lymph. Infections usually begin in childhood.

The skin condition the disease causes is called "elephantiasis tropica" (also known as "elephantiasis arabum").

Elephantiasis mainly affects the lower extremities; the ears, mucous membranes, and amputation stumps are affected less frequently. However, various species of filarial worms tend to affect different parts of the body: "Wuchereria bancrofti" can affect the arms, breasts, legs, scrotum, and vulva (causing hydrocele formation), while "Brugia timori" rarely affects the genitals. Those who develop the chronic stages of elephantiasis are usually amicrofilaraemic and often have adverse immunological reactions to the microfilariae as well as the adult worms.

The subcutaneous worms present with skin rashes, urticarial papules, and arthritis, as well as hyper- and hypopigmentation macules. "Onchocerca volvulus" manifests itself in the eyes, causing "river blindness" (onchocerciasis), one of the leading causes of blindness in the world.

Serous cavity filariasis presents with symptoms similar to subcutaneous filariasis, in addition to abdominal pain, because these worms are also deep-tissue dwellers.

Elephantiasis leads to marked swelling of the lower half of the body.

Zoonoses are infectious diseases of animals (usually vertebrates) that can naturally be transmitted to humans.

Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. "Taenia solium" infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like rabies, are considered direct zoonosis.

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον "zoon" "animal" and νόσος "nosos" "sickness".

Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).

Filariasis such as loiasis most often consists of asymptomatic microfilaremia. Some patients can develop lymphatic dysfunction causing lymphedema. Episodic angioedema (Calabar swellings) in the arms and legs, caused by immune reactions are common. Calabar swellings are 3–10 cm in surface non erythematous and not pitting. When chronic, they can form cyst-like enlargements of the connective tissue around the sheaths of muscle tendons, becoming very painful when moved. The swellings may last for 1–3 days, and may be accompanied by localized urticaria (skin eruptions) and pruritus (itching). They reappear at referent locations at irregular time intervals. Subconjunctival migration of an adult worm to the eyes can also occur frequently, and this is the reason Loa loa is also called the "African eye worm." The passage over the eyeball can be sensed, but it usually takes less than 15 min. Gender incidence of eyeworms have approximately the same frequency, but it tends to increase with age. Eosinophilia is often prominent in filarial infections. Dead worms may cause chronic abscesses, which may lead to the formation of granulomatous reactions and fibrosis.

In the human host, "Loa loa" larvae migrate to the subcutaneous tissue where they mature to adult worms in approximately one year, but sometimes up to four years. Adult worms migrate in the subcutaneous tissues at a speed less than 1 cm/min, mating and producing more microfilariae. The adult worms can live up to 17 years in the human host.

Brugia timori is a human filarial parasitic nematode (roundworm) which causes the disease "Timor filariasis." While this disease was first described in 1965, the identity of "Brugia timori" as the causative agent was not known until 1977. In that same year, "Anopheles barbirostris" was shown to be its primary vector. There is no known animal reservoir host.

Generally speaking, acanthocheilonemiasis does not show initial symptoms. However, if symptoms do arise, it is typically in individuals who are visiting highly infected areas rather than natives to those areas. A major common laboratory finding is an increase in specialized white blood cells, which is called eosinophilia.

Other symptoms include itchy skin, neurological symptoms, abdominal and chest pain, muscle pain, and swelling underneath the skin. If there are abnormally high levels of white blood cells, then a physical examination will most likely find an enlarged spleen or liver.

In certain scenarios, nematodes may physically lodge into the chest or abdomen, resulting in an inflammation. Diagnosis of this condition usually occurs via a blood smear examination under light microscopy.

Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding black flies and mosquitoes. This disease belongs to the group of diseases called helminthiases.

Eight known filarial nematodes use humans as their definitive hosts. These are divided into three groups according to the niche they occupy in the body:

- Lymphatic filariasis is caused by the worms "Wuchereria bancrofti", "Brugia malayi", and "Brugia timori". These worms occupy the lymphatic system, including the lymph nodes; in chronic cases, these worms lead to the syndrome of "elephantiasis".

- Subcutaneous filariasis is caused by "Loa loa" (the eye worm), "Mansonella streptocerca", and "Onchocerca volvulus". These worms occupy the subcutaneous layer of the skin, in the fat layer. "L. loa" causes "Loa loa" filariasis, while "O. volvulus" causes river blindness.

- Serous cavity filariasis is caused by the worms "Mansonella perstans" and "Mansonella ozzardi", which occupy the serous cavity of the abdomen. "Dirofilaria immitis", or the dog heartworm rarely infects humans.

The adult worms, which usually stay in one tissue, release early larval forms known as microfilariae into the host's bloodstream. These circulating microfilariae can be taken up with a blood meal by the arthropod vector; in the vector, they develop into infective larvae that can be transmitted to a new host.

Individuals infected by filarial worms may be described as either "microfilaraemic" or "amicrofilaraemic", depending on whether microfilariae can be found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilariae in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood.

"Loa loa" filariasis is a skin and eye disease caused by the nematode worm "Loa loa". Humans contract this disease through the bite of a deer fly or mango fly ("Chrysops" spp), the vectors for "Loa loa". The adult "Loa loa" filarial worm migrates throughout the subcutaneous tissues of humans, occasionally crossing into subconjunctival tissues of the eye where it can be easily observed. "Loa loa" does not normally affect one's vision but can be painful when moving about the eyeball or across the bridge of the nose. The disease can cause red itchy swellings below the skin called "Calabar swellings". The disease is treated with the drug diethylcarbamazine (DEC), and when appropriate, surgical methods may be employed to remove adult worms from the conjunctiva.

The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.

In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host. In the latter case, the disease may only be defined as a "disease" (which by definition means an illness) in hosts who secondarily become ill after contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.

Wuchereria bancrofti is a human parasitic roundworm that is the major cause of lymphatic filariasis. It is one of the three parasitic worms, together with "Brugia malayi" and "B. timori", that infect the lymphatic system to cause lymphatic filariasis. These filarial worms are spread by a variety of mosquito vector species. "W. bancrofti" is the most prevalent of the three and affects over 120 million people, primarily in Central Africa and the Nile delta, South and Central America, the tropical regions of Asia including southern China, and the Pacific islands. If left untreated, the infection can develop into a chronic disease called elephantiasis. In rare conditions it also causes tropical eosinophilia, an asthmatic disease. There is no commercially available vaccine however high rates of cure have been achieved with various anti-filarial regimens and lymphatic filariasis is the target of the WHO Global Program to Eliminate Lymphatic Filariasis with the aim to eradicate the disease as a public health problem by 2020.

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.

Lymphatic filariasis, also known as elephantiasis, is a human disease caused by parasitic worms known as filarial worms. Most cases of the disease have no symptoms. Some people, however, develop a syndrome called elephantiasis, which is marked by severe swelling in the arms, legs, breasts, or genitals. The skin may become thicker as well, and the condition may become painful. The changes to the body may harm the affected person's social and economic situation.

The worms are spread by the bites of infected mosquitoes. Three types of worms are known to cause the disease: "Wuchereria bancrofti", "Brugia malayi", and "Brugia timori", with "Wuchereria bancrofti" being the most common. These worms damage the lymphatic system. The disease is diagnosed by microscopic examination of blood collected during the night. The blood is typically examined as a smear after being stained with Giemsa stain. Testing the blood for antibodies against the disease may also permit diagnosis. Other roundworms from the same family are responsible for river blindness.

Prevention can be achieved by treating entire groups in which the disease exists, known as mass deworming. This is done every year for about six years, in an effort to rid a population of the disease entirely. Medications used include antiparasitics such as albendazole with ivermectin, or albendazole with diethylcarbamazine. The medications do not kill the adult worms but prevent further spread of the disease until the worms die on their own. Efforts to prevent mosquito bites are also recommended, including reducing the number of mosquitoes and promoting the use of bed nets.

In 2015 about 38.5 million people were infected. About 950 million people are at risk of the disease in 54 countries. It is most common in tropical Africa and Asia. Lymphatic filariasis is classified as a neglected tropical diseases and one of the four main worm infections. The disease results in economic losses of many billions of dollars a year.

A contagious disease is a subset category of transmissible diseases, which are transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes.

Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that carry malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact).

The boundary between contagious and non-contagious infectious diseases is not perfectly drawn, as illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation.

A persistent or recurrent cough that gets aggravated at night, weakness, weight loss and a low fever raises the possible diagnosis of this disease. Some children with this disease may also have enlarged lymph nodes in the neck and elsewhere. Others may cough up a little blood and may also have a wheeze.

Adult worms remain in subcutaneous nodules, limiting access to the host's immune system. Microfilariae, in contrast, are able to induce intense inflammatory responses, especially upon their death. "Wolbachia" species have been found to be endosymbionts of "O. volvulus" adults and microfilariae, and are thought to be the driving force behind most of "O. volvulus" morbidity. Dying microfilariae have been recently discovered to release "Wolbachia" surface protein that activates TLR2 and TLR4, triggering innate immune responses and producing the inflammation and its associated morbidity. The severity of illness is directly proportional to the number of infected microfilariae and the power of the resultant inflammatory response.

Skin involvement typically consists of intense itching, swelling, and inflammation. A grading system has been developed to categorize the degree of skin involvement:

- Acute papular onchodermatitis – scattered pruritic papules

- Chronic papular onchodermatitis – larger papules, resulting in hyperpigmentation

- Lichenified onchodermatitis – hyperpigmented papules and plaques, with edema, lymphadenopathy, pruritus and common secondary bacterial infections

- Skin atrophy – loss of elasticity, the skin resembles tissue paper, 'lizard skin' appearance

- Depigmentation – 'leopard skin' appearance, usually on anterior lower leg

- Glaucoma effect – eyes malfunction, begin to see shadows or nothing

Ocular involvement provides the common name associated with onchocerciasis, river blindness, and may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment, including the retina and optic nerve. The microfilariae migrate to the surface of the cornea. Punctate keratitis occurs in the infected area. This clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time, the entire cornea may become opaque, thus leading to blindness. Some evidence suggests the effect on the cornea is caused by an immune response to bacteria present in the worms.

The skin is itchy, with severe rashes permanently damaging patches of skin.

This is an unusual form of epidemic epilepsy associated with onchocerciasis. This syndrome was first described in Tanzania by Louise Jilek-Aall, a Norwegian psychiatric doctor in Tanzanian practice, during the 1960s. It occurs most commonly in Uganda and South Sudan. It manifests itself in previously healthy 5–15-year-old children, is often triggered by eating or low temperatures and is accompanied by cognitive impairment. Seizures occur frequently and may be difficult to control. The electroencephalogram is abnormal but cerebrospinal fluid (CSF) and magnetic resonance imaging (MRI) are normal or show non-specific changes. If there are abnormalities on the MRI they are usually present in the hippocampus. Polymerase chain reaction testing of the CSF does not show the presence of the parasite.

Mansonelliasis (or mansonellosis) is the condition of infection by the nematode "Mansonella".

The disease exists in Africa and tropical Americas, spread by biting midges or blackflies. It is usually asymptomatic.

Infectious pancreatic necrosis (IPN) is a severe viral disease of salmonid fish. It is caused by infectious pancreatic necrosis virus, which is a member of the Birnaviridae family. This disease mainly affects young salmonids, such as trout or salmon, of less than six months, although adult fish may carry the virus without showing symptoms. Resistance to infection develops more rapidly in warmer water. It is highly contagious and found worldwide, but some regions have managed to eradicate or greatly reduce the incidence of disease. The disease is normally spread horizontally via infected water, but spread also occurs vertically. It is not a zoonosis.

A sharp rise in mortality is often seen (depending on the virulence of the disease). Other clinical signs include abdominal swelling, anorexia, abnormal swimming, darkening of the skin, and trailing of the feces from the vent. On necropsy, internal damage (viral necrosis) to the pancreas and thick mucus in the intestines often is present. Surviving fish should recover within one to two weeks.

Diagnostic methods for the detection of the disease include: characteristic histological pancreatic lesion, PCR, indirect fluorescent antibody testing, ELISA, and virus culture. High virus titers can be isolated from carrier animals.

Tropical (pulmonary) eosinophilia, or TPE, is characterized by coughing, asthmatic attacks, and an enlarged spleen, and is caused by "Wuchereria bancrofti", a filarial infection. It occurs most frequently in India and Southeast Asia. Tropical eosinophilia is considered a manifestation of a species of microfilaria. This disease can be confused with tuberculosis, asthma, or coughs related to roundworms.

Tropical pulmonary eosinophilia is a rare, but well recognised, syndrome characterised by pulmonary interstitial infiltrates and marked peripheral eosinophilia. This condition is more widely recognised and promptly diagnosed in filariasis-endemic regions, such as the Indian subcontinent, Africa, Asia and South America. In nonendemic countries, patients are commonly thought to have bronchial asthma. Chronic symptoms may delay the diagnosis by up to five years. Early recognition and treatment with the antifilarial drug, diethylcarbamazine, is important, as delay before treatment may lead to progressive interstitial fibrosis and irreversible impairment.

The condition of marked eosinophilia with pulmonary involvement was first termed tropical pulmonary eosinophilia in 1950. The syndrome is caused by a distinct hypersensitive immunological reaction to microfilariae of" W. bancrofti" and "Brugia malayi". However, only a small percentage (< 0.5%) of the 130 million people globally who are infected with filariasis apparently develop this reaction. The clearance of rapidly opsonised microfilariae from the bloodstream results in a hypersensitive immunological process and abnormal recruitment of eosinophils, as reflected by extremely high IgE levels of over 1000 kU/L. The typical patient is a young adult man from the Indian subcontinent.