FACTBOX: The world's largest disease outbreaks and threats

(Reuters) -The new swine flu virus that has killed up to 159 people in Mexico and one Mexican baby in the United States remains tiny in scale compared to other global epidemics.

Following are details of the some of the world's largest disease outbreaks and threats:

* HIV/AIDS:

-- An estimated 33 million people worldwide are living with HIV, an immunity-destroying disease that spreads mainly through sexual contact, blood transfusions and needle-sharing. Almost all live in developing countries.

-- Every year about 2.5 million people are newly-infected with the virus and 2 million die from AIDS-related causes. Drugs known as antiretrovirals, taken on a continual basis, have been shown to extend the lifespan of people with the disease.

-- HIV/AIDS has killed more than 25 million people worldwide. It is the leading cause of death in sub-Saharan Africa and the fourth-leading cause of death globally.

* TUBERCULOSIS:

-- One third of the world's population -- more than 2 billion people -- are infected with the bacterium causing tuberculosis, a disease affecting mainly the lungs.

-- About 9 million people develop the disease each year when their immune systems weaken, normally from illness or pregnancy. In 2007 there were 1.3 million tuberculosis deaths among people without HIV, and 456,000 deaths among people infected with both tuberculosis and HIV.

-- Tuberculosis can normally be treated with antibiotics, but drug-resistant forms of the disease have made it increasingly hard, and costly, to treat.

* MALARIA

-- Almost half the world's population, 3.3 billion people, are at risk from malaria, a tropical disease that is transmitted from human to human by mosquitoes.

-- Each year nearly 1 million people die from malaria, mostly children under the age of 5, and 189 million to 327 million are diagnosed with the disease that can be prevented with drugs.

-- Those at highest risk include children, pregnant women, travelers, refugees, and migrant workers in endemic areas.

* INFLUENZA:

-- Between 3 million and 5 million people experience severe illness due to regular, seasonal flu around the world each year, and between 250,000 and 500,000 die as a result.

-- Most deaths in developed economies are among the elderly over the age of 65. Much less is known about the impact of flu in developing countries, though outbreaks in the tropics are thought to have higher fatality rates than those in temperate areas where flu occurs mainly during the winter.

-- There were three pandemic flu outbreaks in the 20th century -- in 1918, 1957 and 1968 -- known respectively as Spanish, Asian, and Hong Kong influenza. An estimated 50 million people died in the first outbreak, about 2 million died in the second and between 1 million and 3 million died in the third.

* CHOLERA:

-- Cholera is an acute diarrhea infection spread most often through contaminated water and food, and through poor hygiene. It can kill healthy adults within hours from severe dehydration.

-- WHO said earlier this year that millions of people are probably infected with cholera each year -- 10 times the number officially reported by countries. As many as one in two people may die without treatment with rehydration salts or antibiotics.

-- More than 4,000 people were killed in a recent Zimbabwe cholera epidemic and a smaller number of people died in Iraq. Contaminated water sources were faulted in both cases.

* HEPATITIS B AND C:

-- About 2 billion people worldwide have been infected with Hepatitis B, a viral infection that attacks the liver. About 600,000 people die each year from its acute or chronic effects.

-- It spreads like HIV, but unlike the AIDS virus Hepatitis B can survive outside the body for at least seven days. It is preventable through vaccination.

-- Hepatitis C infects 3 to 4 million people a year, primarily by direct contact with blood. No vaccine exists for Hepatitis C which is a major cause of acute hepatitis and chronic liver disease, including cirrhosis and liver cancer.

* DENGUE FEVER:

-- The most widespread tropical disease after malaria, dengue is transmitted by mosquitoes and can, in its most severe form, lead to uncontrolled bleeding and death.

-- Some 2.5 billion people -- two fifths of the world's population -- are at risk from the disease that is endemic in more than 100 countries.

-- According to WHO, 50 million people are infected with dengue each year, mainly in urban areas in the tropics and sub-tropics. No specific treatment exists.

* YELLOW FEVER:

-- Named after the jaundice it causes in some patients, yellow fever is a viral disease that causes 30,000 deaths a year.

-- Some 33 African countries, with a combined population of 508 million, are at risk for yellow fever. It is also endemic in nine South American countries and several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru are considered at greatest risk, according to WHO.

* MENINGITIS:

-- Meningitis is a potentially fatal bacterial infection of the thin lining that surrounds the brain and spinal cord. It is most common in sub-Saharan Africa, which had more than 25,000 deaths from the disease in a 1996 outbreak.

-- Vaccines exist to prevent the disease and several aid groups are working to increase their availability in high-risk areas. UNICEF said earlier this month that more than 2,500 people had died from the disease in Nigeria, Niger, Burkina Faso and Chad since the start of 2009.

-- Up to 20 percent of people who survive meningitis infection suffer brain damage, hearing loss, or learning disability. Meningitis normally spreads through kisses, sneezes, coughs, and shared kitchen utensils and drinking cups.

* AFRICAN TRYPANOSOMIASIS (SLEEPING SICKNESS):

-- This parasitic disease, transmitted by tsetse flies, is fatal without treatment and threatens millions of people in 36 countries in sub-Saharan Africa.

-- Major outbreaks have been observed in recent years in Angola, the Democratic Republic of Congo, and Sudan, and it remains a major public health problem in Central African Republic, Chad, Congo, Cote d'Ivoire, Guinea, Malawi, Uganda, and Tanzania.

* MARBURG HAEMORRHAGIC FEVER:

-- A highly fatal disease caused by a virus from the same family as the one causing Ebola hemorrhagic fever, with no vaccine or specific treatment.

-- The biggest known outbreak occurred in Angola in 2004-2005 when 329 people died -- 88 percent of those who were infected with the virus. Another 128 people died in another outbreak in the Democratic Republic of Congo in 1998-2000. Marburg has also been found in Kenya, South Africa, and Uganda.

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(Compiled by Laura MacInnis in Geneva with additional reporting by David Cutler in London)