They cause outbreaks, epidemics, even pandemics that spread from continent to continent. Modern medicine and hygiene have given us some control over devastating infectious diseases, even eradicating smallpox, but, for the most part they remain with us, often preying upon the poorest and most vulnerable.
A photo taken in 1975 shows the village cemetery in the Bangladesh countryside where smallpox victims were buried. The disease was believed to have killed 46 percent of its victims at a hospital in the Dacca, Bangladesh and to have ravaged the country for centuries. A disease marked by lesions on the skin, <a href="http://www.livescience.com/7509-smallpox-changed-world.html">smallpox</a> is believed to have emerged about 3,000 years ago in India or Egypt before sweeping across continents. The variola virus, which causes smallpox, killed as many as third of those it infected and left others scarred and blinded, according to the World Health Organization. In 1980, the WHO declared the disease officially eradicated, after a decade-long vaccination campaign. The last known <a href="http://www.livescience.com/13667-smallpox-virus-destruction-variola-vaccination.html"> remaining samples of the virus </a> are being held in facilities in the U.S. and Russia.
Unlike smallpox, this ancient killer is still with us. Plague, which is caused by a bacterium carried by fleas — like the one shown above — has been blamed for decimating societies including 14th century Europe during the Black Death, when it wiped out roughly a third of the population. The disease comes in three forms, but the best known is <a href="http://www.livescience.com/8914-family-tree-plague-traces-paths-china.html">bubonic plague</a>, which is marked by buboes, or painfully swollen lymph nodes. Plague is now found in animals throughout the world, particularly in the western U.S. and Africa. In 2009, the World Health Organization reported 958 cases worldwide.
Although it is preventable and curable, malaria has a devastating effect in Africa, where the disease accounts for 20 percent of all childhood deaths, according to the World Health Organization. It is present on other continents as well. A parasite carried by blood-sucking mosquitoes causes the disease, which is first characterized by fever, chills and flu-like symptoms before progressing on to more serious complications. By 1951, the disease was eliminated from the U.S. with the help of the pesticide DDT. A subsequent WHO campaign to eradicate malaria was successful only in some places, and the goal was downgraded to reducing transmission of disease, according to the U.S. Centers for Disease Control and Prevention.
This 3-D model illustrates a generic flu virus (there are different types). A seasonal, respiratory infection, flu is responsible for about three to five million cases of severe illness, and about 250,000 to 500,000 deaths, according to the World Health Organization. Periodically, however, the viral infection becomes much more devastating: A pandemic in 1918 killed about 50 million people worldwide. As we learned from "swine flu" and "bird flu" scares in recent years, some influenza viruses can jump between species.
Potentially fatal "TB" is caused by the bacterium Mycobacterium tuberculosis, which usually attacks the lungs causing the signature bloody coughs. The x-ray above shows the chest of a patient suffering from far-advanced tuberculosis. The bacterium does not make everyone it infects sick, and up to one third of the world's population currently carries the bacterium. And among people infected with TB, but not HIV, 5 to 10 percent become sick or infectious at some time during their lifetimes. A full-blown TB infection is more common among those also infected with HIV. The TB bacterium has formed a deadly alliance with the immune-system-destroying HIV, with each disease worsening the other, according to the World Health Organization.
In 2009, about 33 million people were living with a Human Immunodeficiency Virus infection and about 1.8 million died from it, according to the World Health Organization. While many of the worst offenders have a longstanding relationship with humans, HIV is a recent arrival. HIV's decimating effect on certain immune system cells was first documented in 1981. By destroying part of the immune system, HIV leaves its victims vulnerable to all sorts of opportunistic diseases. It is believed to have emerged from Simian Immunodeficiency Virus (SIV) which infects apes and monkeys.
Cholera causes acute diarrhea and, if left untreated, a severe infection can kill within hours. The bacterium responsible is usually found in food or water contaminated by infected feces. Thanks to improved sanitation, cholera has been rare in industrialized nations for the last 100 years. During the 19th century, cholera spread from its home in India, causing six pandemics that killed millions of people on all continents, according to the World Health Organization. This waiting room in Peruvian hospital was converted to an emergency cholera ward during an epidemic in 1992.