The tropics include portions of nearly every continent and some of the world’s most populous countries. The number of diseases endemic to such a large area with so many people is difficult to estimate. To better understand them, they may be categorized by the organism that causes the disease.

Viruses are tiny parasites that often exist as inactive spores outside of a host organism. Once inside a plant, animal or bacteria cell, they are able to replicate and may cause disease. The tropical diseases they cause include the following:

• Dengue fever and dengue hemorrhagic fever. These two diseases are caused by four viruses that typically occur in urban areas in Asia, Africa, the South Pacific, the Caribbean, and Central and South America. The viruses transmit from person to person by mosquito bites and cause 100 million cases annually, according to the U.S. Centers for Disease Control and Prevention (CDC). Patients who have had dengue fever previously are more likely to develop the more serious dengue hemorrhagic fever if infected later in life. Overall, these diseases pose a low risk to travelers unless an epidemic is in progress in the area.
  

  

• Hemorrhagic fever. This group of more than a dozen viral diseases causes illness throughout the tropics. In addition to dengue hemorrhagic fever, they include yellow fever, Lassa fever, Ebola hemorrhagic fever and Rift Valley fever. The number of annual infections varies widely and mortality can range from less than 1 percent of those infected with hemorrhagic fever with renal syndrome to nearly 90 percent of people who develop Ebola. Outside of periodic outbreaks, the chance for travelers being infected with most hemorrhagic fevers is low.

Bacteria can have a more complex effect on humans than viruses. These one-celled organisms may live freely in the environment or within a plant or animal. While some are necessary for the human body to function, others may cause disease. Tropical diseases caused by bacteria include:

• Typhoid fever. Salmonella Typhi (also known as Salmonella enterica serovar Typhi) is responsible for typhoid fever. The CDC estimates 12 million to 22 million cases and 200,000 deaths occur internationally each year. Among those, 400 occur in the United States, mostly among recently returned travelers. The bacteria are expelled through the feces of infected people. Without proper sanitation, they can be spread through contaminated food or water. Outside the body, the bacteria can also be found in bodies of water contaminated with sewage.
  
  
• Cholera. This disease is widespread and is often found outside the tropics. It occurs in Indonesia, throughout much of Asia, Eastern Europe, Africa, the Iberian Peninsula (Spain and Portugal) and Peru. In 2003, more than 100,000 cases in 45 countries were reported to the World Health Organization (WHO). Much like typhoid fever, it is spread by eating contaminated foods or beverages. After infection, patients may show no or only mild symptoms but may still transmit the disease in their feces. The bacteria can also live in contaminated brackish (slightly salty) water and infect shellfish. In general there is a low risk for travelers who avoid eating raw or undercooked foods.
  
  
• Buruli ulcer. Little is known about this disease caused by Mycobacterium ulcerans infection. The number of cases of Buruli ulcer is uncertain, but it occurs in 30 countries in sub-Saharan Africa, North and South America, Australia, and Asia. Scientists are also unsure of how it is transmitted, but it is often associated with swimming in rivers and skin injury and may be carried by aquatic insects, fish or mosquitoes. Most cases are reported in children younger than 15 but it may occur at any age.
  
  
• Leprosy or Hansen disease. Mycobacterium leprae is responsible for this rare disease endemic to 15 countries in Africa, Asia and Latin America. In 2002, the CDC estimated more than 750,000 cases in the world, with fewer than 100 in the United States. Transmission typically requires close or long-term contact with an infected person, making it a very low risk to travelers.

Like bacteria, protozoa are also single-celled organisms. They have a more complex structure, including a nucleus (the core of a cell containing chromosomes) and are often classified within the animal kingdom. Some may infect humans and cause disease. In the tropics, these diseases include:

• Malaria. Parasites of the Plasmodium genus spread by mosquitos are responsible for malaria in humans. Four species have long been known to cause variations of the disease, and a fifth has recently been discovered that was once thought to only affect monkeys. They are endemic to more than 100 countries and may infect up to 500 million people each year, according to the WHO.  More than 1 million people die from malaria every year. Travelers are at risk of being infected if prophylactic, or preventative, medications are not taken. In addition, preventative medications are recommended for travel to all areas where people may be exposed to malaria. For example, recent travelers to the Caribbean countries of the Bahamas and the Dominican Republic have contracted malaria even though it is not endemic to these areas.
  

  

• Leishmaniasis. This disease is caused by more than 20 species of protozoa that are spread by sand flies. According to the CDC, there are approximately 2 million new cases each year in 88 countries. The majority occur in India, Bangladesh, Nepal, Sudan and Brazil. Leishmaniasis is also found throughout much of the Americas, including occasionally in south-central Texas. The disease occurs most often in rural settings and rarely infects travelers.
  
  
• African sleeping sickness. The tsetse fly is responsible for spreading the two Trypanosoma parasites that cause both forms of this disease (East African and West African). Each year, up to 500,000 people are infected in 36 sub-Saharan countries in Africa, according to CDC estimates. Of those people, 50,000 die, according to the WHO. Of the two varieties, East African sleeping sickness is more severe and is also responsible for most cases reported in international travelers.
  
  
• Chagas disease or American trypanosomiasis. Like African sleeping sickness, Chagas disease is also caused by a parasite of the genus Trypanosoma. According to CDC estimates, it infects 11 million to 18 million people annually in 18 countries. The protozoa are occasionally found in the United States, but normally range from Mexico through Central and South America. The parasites are spread in the fecal matter of triatomine bugs, which are often found in the walls of substandard housing in areas where the disease is endemic. While it rarely directly causes death in adults, the WHO estimates Chagas infection lowers life expectancy on average by nine years.

Parasitic worms may also cause disease in humans. These include many species of roundworms (nematodes) and flatworms (trematodes or flukes). In the tropics, diseases caused by these worms include:

• Schistosomiasis or bilharzia. Five species of parasitic trematodes are responsible for this disease that is endemic to 74 countries. It has infected 200 million people, according to the CDC, with most cases occurring in sub-Saharan Africa. The parasites are also found in South America, the Caribbean, the Middle East, southern China and Southeast Asia. The parasite spends part of its life in snails inhabiting bodies of water contaminated with urine or feces of infected humans. The larval form of the worm leaves its snail host and is free-swimming. If a person contacts the water (e.g., swimming, bathing), the larvae are able to penetrate skin and grow in the blood vessels. The eggs they lay move through the body and may cause severe disease.
  
  
• Filariasis or lymphatic filariasis. This disease is caused by three species of nematodes found in South America, the Caribbean, Africa, Asia and the islands of the Western Pacific. According to the WHO, 120 million people in 80 countries have been infected around the world, with approximately 40 percent of cases occurring in India. While rarely fatal, filariasis is a leading global cause of long-term disability. The larval form of the parasite is spread by mosquitoes from person to person. Once inside the body, they move into the patient’s lymphatic system, where they may cause fluid to collect. For the parasites to reproduce and cause disease, the patient must be infected repeatedly over months to years. This limits the risk to short-term travelers.
  
  
• River blindness or onchocerciasis. Like filariasis, river blindness is caused by a parasitic nematode. The WHO estimates this disease infects a total of 17.7 million people, causing 270,000 to go blind. Of the people infected, 99 percent are from 28 African countries. The remainder of the cases occur in Yemen, Mexico, Guatemala, Ecuador, Colombia, Venezuela and Brazil. Black flies spread the larval form of the parasite and the severity of the disease is often related to the number of times the patient is bitten.