When properly treated, people with malaria can usually expect a complete recovery. However, severe malaria can progress extremely rapidly and cause death within hours or days. In the most severe cases of the disease, fatality rates can reach 20%, even with intensive care and treatment. Over the longer term, developmental impairments have been documented in children who have suffered episodes of severe malaria. Chronic infection without severe disease can occur in an immune-deficiency syndrome associated with a decreased responsiveness to "Salmonella" bacteria and the Epstein–Barr virus.

During childhood, malaria causes anemia during a period of rapid brain development, and also direct brain damage resulting from cerebral malaria. Some survivors of cerebral malaria have an increased risk of neurological and cognitive deficits, behavioural disorders, and epilepsy. Malaria prophylaxis was shown to improve cognitive function and school performance in clinical trials when compared to placebo groups.

Malaria parasites belong to the genus "Plasmodium" (phylum Apicomplexa). In humans, malaria is caused by "P. falciparum", "P. malariae", "P. ovale", "P. vivax" and "P. knowlesi". Among those infected, "P. falciparum" is the most common species identified (~75%) followed by "P. vivax" (~20%). Although "P. falciparum" traditionally accounts for the majority of deaths, recent evidence suggests that "P. vivax" malaria is associated with potentially life-threatening conditions about as often as with a diagnosis of "P. falciparum" infection. "P. vivax " proportionally is more common outside Africa. There have been documented human infections with several species of "Plasmodium" from higher apes; however, except for "P. knowlesi"—a zoonotic species that causes malaria in macaques—these are mostly of limited public health importance.

Global warming is likely to affect malaria transmission, but the severity and geographic distribution of such effects is uncertain.

Globally, an estimated 125 million or more pregnant women per year risk contracting PAM. Pregnancy-related malaria causes around 100,000 infant deaths each year, due in large part to low birth weight.

The disease results from the aggregation of erythrocytes infected by "Plasmodium falciparum" which have been shown to adhere to chondroitin sulfate A (CSA) on placental proteoglycans causing them to accumulate in the intervillous spaces of the placenta, blocking the crucial flow of nutrients from mother to embryo.

It is estimated that a third of all pregnant women in developing countries are infected with hookworm, 56% of all pregnant women in developing countries suffer from anemia, 20% of all maternal deaths are either directly or indirectly related to anemia. Numbers like this have led to an increased interest in the topic of hookworm-related anemia during pregnancy. With the understanding that chronic hookworm infection can often lead to anemia, many people are now questioning if the treatment of hookworm could effect change in severe anemia rates and thus also on maternal and child health as well. Most evidence suggests that the contribution of hookworm to maternal anemia merits that all women of child-bearing age living in endemic areas be subject to periodic anthelmintic treatment. The World Health Organization even recommends that infected pregnant women be treated after their first trimester. Regardless of these suggestions, only Madagascar, Nepal and Sri Lanka have added deworming to their antenatal care programs.

This lack of deworming of pregnant women is explained by the fact that most individuals still fear that anthelmintic treatment will result in adverse birth outcomes. But a 2006 study by Gyorkos et al. found that when comparing a group of pregnant women treated with mebendazole with a control placebo group, both illustrated rather similar rates in adverse birth outcomes. The treated group demonstrated 5.6% adverse birth outcomes, while the control group had 6.25% adverse birth outcomes. Furthermore, Larocque et al. illustrated that treatment for hookworm infection actually led to positive health results in the infant. This study concluded that treatment with mebendazole plus iron supplements during antenatal care significantly reduced the proportion of very low birth weight infants when compared to a placebo control group. Studies so far have validated recommendations to treat infected pregnant women for hookworm infection during pregnancy.

A review of effects of antihelminthics (anti-worm drugs) given in pregnancy found that there was not enough evidence to support treating pregnant women in their second or third trimesters. The women who were treated in the second trimester and the women who had no treatment showed no difference in numbers of maternal anemia, low birth weight, preterm birth or deaths of babies.

The intensity of hookworm infection as well as the species of hookworm have yet to be studied as they relate to hookworm-related anemia during pregnancy. Additionally, more research must be done in different regions of the world to see if trends noted in completed studies persist.

Mosquito-borne diseases, such as dengue fever and malaria, typically affect third world countries and areas with tropical climates. Mosquito vectors are sensitive to climate changes and tend to follow seasonal patterns. Between years there are often dramatic shifts in incidence rates. The occurrence of this phenomenon in endemic areas makes mosquito-borne viruses difficult to treat.

Dengue fever is caused by infection through viruses of the family Flaviviridae. The illness is most commonly transmitted by Aedes aegypti mosquitoes in tropical and subtropical regions. Dengue virus has four different serotypes, each of which are antigenically related but have limited cross-immunity to reinfection.

Although dengue fever has a global incidence of 50-100 million cases, only several hundreds of thousands of these cases are life-threatening. The geographic prevalence of the disease can be examined by the spread of the Aedes aegypti. Over the last twenty years, there has been a geographic spread of the disease. Dengue incidence rates have risen sharply within urban areas which have recently become endemic hot spots for the disease. The recent spread of Dengue can also be attributed to rapid population growth, increased coagulation in urban areas, and global travel. Without sufficient vector control, the dengue virus has evolved rapidly over time, posing challenges to both government and public health officials.

Malaria is caused by a protozoan called Plasmodium falciparum. P. falciparum parasites are transmitted mainly by the Anopheles gambiae complex in rural Africa. In just this area, P. falciparum infections comprise an estimated 200 million clinical cases and 1 million annual deaths. 75% of individuals afflicted in this region are children. As with dengue, changing environmental conditions have led to novel disease characteristics. Due to increased illness severity, treatment complications, and mortality rates, many public health officials concede that malaria patterns are rapidly transforming in Africa. Scarcity of health services, rising instances of drug resistance, and changing vector migration patterns are factors that public health officials believe contribute to malaria’s dissemination.

Climate heavily affects mosquito vectors of malaria and dengue. Climate patterns influence the lifespan of mosquitos as well as the rate and frequency of reproduction. Climate change impacts have been of great interest to those studying these diseases and their vectors. Additionally, climate impacts mosquito blood feeding patterns as well as extrinsic incubation periods. Climate consistency gives researchers an ability to accurately predict annual cycling of the disease but recent climate unpredictability has eroded researchers’ ability to track the disease with such precision.

Co-infection with hookworm and "Plasmodium falciparum" is common in Africa. Although exact numbers are unknown, preliminary analyses estimate that as many as a quarter of African schoolchildren (17.8–32.1 million children aged 5–14 years) may be coincidentally at-risk of both "P. falciparum" and hookworm. While original hypotheses stated that co-infection with multiple parasites would impair the host’s immune response to a single parasite and increase susceptibility to clinical disease, studies have yielded contrasting results. For example, one study in Senegal showed that the risk of clinical malaria infection was increased in helminth-infected children in comparison to helminth-free children while other studies have failed to reproduce such results, and even among laboratory mouse experiments the effect of helminths on malaria is variable. Some hypotheses and studies suggest that helminth infections may protect against cerebral malaria due to the possible modulation of pro-inflammatory and anti-inflammatory cytokines responses. Furthermore, the mechanisms underlying this supposed increased susceptibility to disease are unknown. For example, helminth infections cause potent and highly polarized immune response characterized by increased T-helper cell type 2 (T2) cytokine and Immunoglobulin E(IgE) production. However, the effect of such responses on the human immune response is unknown. Additionally, both malaria and helminth infection can cause anemia, but the effect of co-infection and possible enhancement of anemia is poorly understood.

The arboviruses have expanded their geographic range and infected populations that had no recent community knowledge of the diseases carried by the "Aedes aegypti" mosquito. Education and community awareness campaigns are necessary for prevention to be effective. Communities are educated on how the disease is spread, how they can protect themselves from infection and the symptoms of infection. Community health education programs can identify and address the social/economic and cultural issues that can hinder preventative measures. Community outreach and education programs can identify which preventative measures a community is most likely to employ. Leading to a targeted prevention method that has a higher chance of success in that particular community. Community outreach and education includes engaging community health workers and local healthcare providers, local schools and community organizations to educate the public on mosquito vector control and disease prevention.

Cryptosporidiosis is found worldwide. It causes 50.8% of water-borne diseases that are attributed to parasites. In developing countries, 8–19% of diarrheal diseases can be attributed to "Cryptosporidium". Ten percent of the population in developing countries excretes oocysts. In developed countries, the number is lower at 1–3%. The age group most affected are children from 1 to 9 years old.

Roughly 30% of adults in the United States are seropositive for cryptosporidiosis, meaning that they contracted the infection at some point in their lives.

Some of the strategies for controlling tropical diseases include:

- Draining wetlands to reduce populations of insects and other vectors, or introducing natural predators of the vectors.

- The application of insecticides and/or insect repellents) to strategic surfaces such as clothing, skin, buildings, insect habitats, and bed nets.

- The use of a mosquito net over a bed (also known as a "bed net") to reduce nighttime transmission, since certain species of tropical mosquitoes feed mainly at night.

- Use of water wells, and/or water filtration, water filters, or water treatment with water tablets to produce drinking water free of parasites.

- Sanitation to prevent transmission through human waste.

- In situations where vectors (such as mosquitoes) have become more numerous as a result of human activity, a careful investigation can provide clues: for example, open dumps can contain stagnant water that encourage disease vectors to breed. Eliminating these dumps can address the problem. An education campaign can yield significant benefits at low cost.

- Development and use of vaccines to promote disease immunity.

- Pharmacologic pre-exposure prophylaxis (to prevent disease before exposure to the environment and/or vector).

- Pharmacologic post-exposure prophylaxis (to prevent disease after exposure to the environment and/or vector).

- Pharmacologic treatment (to treat disease after infection or infestation).

- Assisting with economic development in endemic regions. For example, by providing microloans to enable investments in more efficient and productive agriculture. This in turn can help subsistence farming to become more profitable, and these profits can be used by local populations for disease prevention and treatment, with the added benefit of reducing the poverty rate.

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Additional neglected tropical diseases include:

Some tropical diseases are very rare, but may occur in sudden epidemics, such as the Ebola hemorrhagic fever, Lassa fever and the Marburg virus. There are hundreds of different tropical diseases which are less known or rarer, but that, nonetheless, have importance for public health.

Infection is through contaminated material such as earth, water, uncooked or cross-contaminated food that has been in contact with the feces of an infected individual or animal. Contact must then be transferred to the mouth and swallowed. It is especially prevalent amongst those in regular contact with bodies of fresh water including recreational water such as swimming pools. Other potential sources include insufficiently treated water supplies, contaminated food, or exposure to feces. The high resistance of "Cryptosporidium" oocysts to disinfectants such as chlorine bleach enables them to survive for long periods and still remain infective. Some outbreaks have happened in day care related to diaper changes.

The following groups have an elevated risk of being exposed to "Cryptosporidium":

- Child care workers

- Parents of infected children

- People who take care of other people with cryptosporidiosis

- International travelers

- Backpackers, hikers, and campers who drink unfiltered, untreated water

- People, including swimmers, who swallow water from contaminated sources

- People who handle infected cattle

- People exposed to human feces through sexual contact

Cases of cryptosporidiosis can occur in a city that does not have a contaminated water supply. In a city with clean water, it may be that cases of cryptosporidiosis have different origins. Testing of water, as well as epidemiological study, are necessary to determine the sources of specific infections. "Cryptosporidium" is causing serious illness more frequently in immunocompromised than in apparently healthy individuals. It may chronically sicken some children, as well as adults who are exposed and immunocompromised. A subset of the immunocompromised population is people with AIDS. Some sexual behaviors can transmit the parasite directly.

Tropical and sub-tropical regions are the main areas affected by nematodes and parasitic worms, which often causes filariasis. Around 20% of immigrants to Spain are children from these regions. There are concerns about absolute eosinophilia in immigrants that is correlated with parasitic diseases that may go undiagnosed. Absolute eosinophilia is clinically diagnosed as >0.45×10 eosinophilic leucocytes/L of peripheral blood. Recent studies suggest that around 60% of children with relative eosinophilia contracted this via parasitic infections. Relative eosinophilia is different from absolute because relative refers to an increase in percentage of white blood cells (i.e. leukocytes) due to a loss of blood plasma; where as absolute eosinophilia is purely an increase in white blood cell production. Of those with relative eosinophilia, 40% were undiagnosed until these studies. Therefore, there is a great need for thorough parasitological studies in this area of tropical infectious diseases.

They are treated with antiprotozoal agents. Recent papers have also proposed the use of viruses to treat infections caused by protozoa.

Acanthocheilonemiasis is caused by the parasite, "mansonella perstans." "M. perstans" is primarily found in central Africa and in some areas of South America, therefore the most affected populations are located in these areas. Acanthocheilonemiasis affects humans in these areas in equal numbers. The prevalence of this condition does significantly increase with age. Furthermore, the parasite is most commonly found in areas of tropical forests with alternating swamps and open ground.

Approximately 114 million people in Africa are infected with "M. perstans", including 33 sub-Saharan African countries. Recent studies focused on Gabon specifically, where febrile and tropical diseases are common. Contrary to popular recent suggestions, "M. perstans" does not influence the emergence of febrile diseases, including HIV, tuberculosis, bacteremia, and malaria. In general, hemoglobin levels in individuals with malaria are severely reduced from that of a healthy individual. Reduced levels occur because the malaria parasite, "Plasmodium falciparum," utilizes human hemoglobin as its major energy source. Filariasis, in combination with severe malaria, actually shows higher hemoglobin levels than in severe malaria alone. In addition, "M. perstans" did not have adverse effects on those with HIV, as there were actually higher levels of CD4 in HIV patients co-infected with "M. perstans". Further research in this area may allude to clinical manifestations of this infectious disease, as there could be possible benefits by contracting "M. perstans".

Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in all parts of the world apart from Antarctica. There are several other species of "Plasmodium" that infect birds, such as "Plasmodium anasum" and "Plasmodium gallinaceum", but these are of less importance except, in occasional cases, for the poultry industry. The disease is found worldwide, with important exceptions. Usually, it does not kill birds. However, in areas where avian malaria is newly introduced, such as the islands of Hawaiʻi, it can be devastating to birds that have lost evolutionary resistance over time.

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).

Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera "Plasmodium" and "Hemoproteus" (phylum Apicomplexa, class Haemosporidia, family Plasmoiidae). The disease is transmitted by a dipteran vector including mosquitoes in the case of "Plasmodium" parasites and biting midges for "Hemoproteus." The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.

Mammals can get parasites from contaminated food or water, bug bites, or sexual contact. Ingestion of contaminated water can produce Giardia infections.

Parasites normally enter the body through the skin or mouth. Close contact with pets can lead to parasite infestation as dogs and cats are host to many parasites.

Other risks that can lead people to acquire parasites are walking barefeet, inadequate disposal of feces, lack of hygiene, close contact with someone carrying specific parasites, and eating undercooked foods, unwashed fruits and vegetables or foods from contaminated regions.

Parasites can also be transferred to their host by the bite of an insect vector, i.e. mosquito, bed bug, fleas.

Estimates of the rate of HCV vertical transmission range from 2–8%; a 2014 systematic review and meta-analysis found the risk to be 5.8% in HCV-positive, HIV-negative women. The same study found the risk of vertical transmission to be 10.8% in HCV-positive, HIV-positive women. Other studies have found the risk of vertical transmission to be as high as 44% among HIV-positive women. The risk of vertical transmission is higher when the virus is detectable in the mother's blood.

Evidence does not indicate that mode of delivery (i.e. vaginal vs. cesarean) has an effect on vertical transmission.

For women who are HCV-positive and HIV-negative, breastfeeding is safe; however, CDC guidelines suggest avoiding breastfeeding if a woman's nipples are "cracked or bleeding" to reduce the risk of 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.

"Babesia" parasites reproduce in red blood cells, where they can be seen as cross-shaped inclusions (four merozoites asexually budding, but attached together forming a structure looking like a "Maltese cross") and cause hemolytic anemia, quite similar to malaria.

Unlike the "Plasmodium" parasites that cause malaria, "Babesia" species lack an exoerythrocytic phase, so the liver is usually not affected.

In nonhuman animals, "Babesia canis rossi", "Babesia bigemina", and "Babesia bovis" cause particularly severe forms of the disease, including a severe haemolytic anaemia, with positive erythrocyte-in-saline-agglutination test indicating an immune-mediated component to the haemolysis. Common sequelae include haemoglobinuria "red-water", disseminated intravascular coagulation, and "cerebral babesiosis" caused by sludging of erythrocytes in cerebral capillaries.

In bovine species, the organism causes hemolytic anemia, so an infected animal shows pale mucous membranes initially. As the levels of bilirubin (a byproduct of red blood cell lysis) continue to increase, the visible mucous membranes become yellow in color (icterus) due to the failure of the liver to metabolize the excess bilirubin. Hemoglobinuria is seen due to excretion of red-blood-cell lysis byproducts via the kidneys. Fever of 40.5 °C (105 °F) develops due to release of inflammatory byproducts.

Treatment of asymptomatic carriers should be considered if parasites are still detected after 3 months. In mild-to-moderate babesiosis, the treatment of choice is a combination of atovaquone and azithromycin. This regimen is preferred to clindamycin and quinine because side effects are fewer. The standard course is 7 to 10 days, but this is extended to at least 6 weeks in people with relapsing disease. Even mild cases are recommended to be treated to decrease the chance of inadvertently transmitting the infection by donating blood. In life-threatening cases, exchange transfusion is performed. In this procedure, the infected red blood cells are removed and replaced with uninfected ones.

Imizol is a drug used for treatment of babesiosis in dogs.

Extracts of the poisonous, bulbous plant "Boophone disticha" are used in the folk medicine of South Africa to treat equine babesiosis. "B. disticha" is a member of the daffodil family Amaryllidaceae and has also been used in preparations employed as arrow poisons, hallucinogens, and in embalming. The plant is rich in alkaloids, some of which display an action similar to that of scopolamine.

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.

Early onset sepsis can occur in the first week of life. It usually is apparent on the first day after birth. This type of infection is usually acquired before the birth of the infant. Premature rupture of membranes and other obstetrical complications can add to the risk of early-onset sepsis. If the amniotic membrane has been ruptured greater than 18 hours before delivery the infant may be at more risk for this complication. Prematurity, low birth weight, chorioamnionitis, maternal urinary tract infection and/or maternal fever are complications that increase the risk for early-onset sepsis. Early onset sepsis is indicated by serious respiratory symptoms. The infant usually suffers from pneumonia, hypothermia, or shock. The mortality rate is 30 to 50%.