• Cholera
• Hepatitis B
• HIV/AIDS
• Influenza
• Malaria
• Plague
• Smallpox
• Tuberculosis
• Yellow Fever

Cholera: The disease is endemic to Africa. In 1970, cholera broke out in West Africa after having disappeared from the planet for 100 years. In 1991, cholera also struck Peru, quickly escalating into an epidemic through most of Latin America. In endemic areas, cholera is mainly a disease of young children, and fatality rates can be as high as 50 percent among children who are afflicted. Limited stocks of two vaccines that provide protection for several months have recently become available in a few countries, although public health authorities don't recommend vaccination to prevent or control outbreaks. According to the World Health Organization, the best prevention methods are improved hygiene around food preparation, safer drinking water supplies and sanitary disposal of human feces. In severe cases, antibiotics reduce the symptoms of cholera, but the disease is increasingly drug-resistant.

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Hepatitis B: This disease, the most serious type of viral hepatitis, is responsible for 1 million deaths each year. Of the 2 billion people who have been infected with the hepatitis B virus worldwide, more than 350 million have lifelong chronic infection. Those who are victims of chronic hepatitis B are at high risk of death from cirrhosis of the liver and liver cancer. In sub-Saharan Africa, in most of Asia and in the Pacific Islands, the majority of hepatitis B sufferers become infected with the virus during childhood. An estimated 10 percent become chronically infected. A hepatitis B vaccine was developed in 1982, and although it does not cure chronic hepatitis, it is 95 percent effective in preventing chronic infections from developing in those who have been exposed to the disease. Hepatitis B is transmitted by contact with blood or bodily fluids of an infected person in the same way as HIV -- primarily through contaminated needles or sexual intercourse -- but it is 50 to 100 times more infectious than HIV.

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HIV/AIDS: Since the start of the global epidemic in the 1980s, HIV has infected almost 58 million men, women and children. AIDS has cost the lives of nearly 22 million adults and children. Despite an intense international campaign against HIV/AIDS for the last few decades, researchers have yet to develop a vaccine. A number of new drug therapies offer new hope to those living with the disease in wealthy countries, including in the United States, where AIDS-related deaths dropped two-thirds between 1997 and 1999. But in the developing world, where 95 percent of all new HIV infections are occurring, the new medications are costly and scarce. In Africa, where AIDS is the leading cause of death, 80 percent of AIDS patients die within two years of becoming infected with HIV.

Evidence suggests that a new HIV strain, one resistant to many drugs, may now be emerging. HIV's ability to mutate has raised fears among some AIDS researchers that the virus could evolve into a more contagious form.

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Influenza: The flu rapidly spreads around the world in seasonal epidemics, killing about 250,000 people every year. The elderly are among those at highest risk of death from flu. Flu vaccines have been available for many years, but because influenza strains are prone to rapid mutation, a new flu vaccine is produced nearly every year.

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Malaria: The World Health Organization announced in 1955 that malaria had been nearly wiped out. But the possibility of total eradication of malaria slipped away over the last half-century. More than 40 percent of the world's population is at risk of infection, and there are more than 1 million malaria deaths every year, more than half of which occur among children. Malaria is endemic to Latin America, Africa and Asia, where resistance to various treatments is also growing. The rise in malaria cases is linked with global warming -- infected mosquitoes breed faster and bite more often in warmer weather. Most efforts against the spread of malaria in the 20th century focused on mosquito control, rather than on a vaccine against plasmodia (the tiny parasites that infected mosquitoes pass on when they bite).

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Plague: More than 2,500 cases of plague, including about 200 deaths, are still reported every year. The majority of cases and deaths are reported from Africa, but the disease is also endemic to some countries in the Americas and Asia. Antibiotics and other treatments available today enable almost all plague patients to be cured if they're diagnosed in time. If plague goes undiagnosed, and therefore untreated, up to 60 percent of those who are infected die. Plague vaccines are available worldwide, but vaccination is only recommended for health workers and laboratory personnel who are regularly exposed to the risk of contamination. Plague prevention includes surveillance of local rodent and flea species.

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Smallpox: By 1900, smallpox -- one of the oldest diseases known to humans -- had been virtually wiped out in North America and Europe, but in the developing world it continued its deadly work, killing more than half a billion people over the course of the 20th century. Although the disease has been nearly eradicated -- the last reported case of smallpox occurred in 1977 -- the virus has not entirely disappeared. Vials of it are still held in laboratories around the world. Many scientists favor destroying the last samples of smallpox because of the potential risk of their being used in biological terrorism.

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Tuberculosis: Tuberculosis kills 2 million people every year. Drugs discovered in the 1940s helped dramatically in beating back TB, and by the 1980s, public health authorities were so optimistic that the World Health Organization (WHO) launched a campaign to eradicate it entirely. But in the last two decades, the disease has proven resistant to modern treatments. AIDS also has been a complicating factor. When HIV weakens the body's immune system, its victims are more vulnerable to tuberculosis. Today, WHO has identified 16 countries in sub-Saharan Africa, Southeast Asia, and Eastern and Pacific Asia as tuberculosis trouble spots. These TB hot spots account for 80 percent of the world's 8 million new cases every year.

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Yellow Fever: There are an estimated 200,000 cases of yellow fever, including 30,000 deaths, each year. But because public health officials suspect underreporting, the numbers could be even higher. A yellow fever vaccine has been available for 60 years. However, with deforestation and urbanization placing human populations closer to mosquito habitats, the number of people infected has risen during the last two decades. Most yellow fever cases occur in Africa and South America, where yellow fever is endemic to nine countries. There is no specific treatment for the disease. Dehydration and fever can be corrected with rehydration salts taken orally. Intensive supportive care may improve the outcome for seriously ill patients, but this kind of care is rarely available in poorer, developing countries.

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Return to Epidemics Through Time: Follow the spread of some of the worst outbreaks of infectious disease

Kelly Whalen is a writer and documentary producer based in Oakland, Calif.