The swine flu is just one of many deadly diseases that have jumped from animals to humans.
Bacteria and viruses that are deadly to one type of creature can evolve quickly to infect another. The cross-species infection can originate on farms or in markets, where conditions foster mixing of pathogens, giving them opportunities to swap genes and gear up to kill previously foreign hosts, like you. Microbes of two varieties can even gather in your gut, do some viral dancing, and evolve to morph you into a deadly, contagious host.
Diseases passed from animals to humans are called zoonoses, and researchers say they are on the rise. Here are five that have had tremendous impact on us:
5. Gorillas Gave Humans 'The Crabs'
Humans caught pubic lice from gorillas about 3 million years ago. We likely picked up the delightful disease, affectionately known as "crabs," not by sleeping with gorillas, but by sleeping in gorilla nests or eating the gorillas, scientists concluded in 2007. Humans, by the way, are the only primates that have both pubic lice and head lice (chimps have just head lice, and you now know which kind gorillas have).
4. Insane Mind Parasite
The bizarre parasite Toxoplasma gondii infects the brains of more than half the human population, including about 50 million Americans. It is thought increase the risk of neuroticism and may contribute to schizophrenia. However, its primary host is house cats. You can get it from cat feces. Initially, symptoms in humans are typically flu-like. But this bug never goes away. Some scientists think it has altered human behavior enough to shape entire cultures. Countries with high prevalence of T. gondii infection also have higher average neuroticism scores, one study found.
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HIV, the virus that causes AIDS, originated from chimps and other primates and is thought to have first infected humans at least a century ago. At the end of 2007, an estimated 33 million people had HIV, including about 2.7 million new cases for the year, and about 2 million died (including 270,000 children) during the year. Two-thirds of HIV infections are in sub-Saharan Africa.
2. The Bubonic Plague
Nothing beats the 14-century Black Death (also called Bubonic Plague) for sheer global impact of a single disease outbreak and bringing civilization to its knees. Corpses piled in the streets from Europe to Egypt and across Asia. Some 75 million died — at a time when there were only about 360 million to start with. Death came in a matter of days, and it was excruciatingly painful. Plague is a bacterial disease caused by Yersinia pestis. It is carried by rodents and even cats, but becomes most deadly to us when transmitted between people, as became the case in the 1300s. It took centuries for some societies to recover, as some of the survivors mistrusted local authorities and in some cases even God, under whose wrath they presumably had suffered.
1. Influenza Pandemics
The swine flu outbreaks cropping up in several countries now are nothing — so far — compared to historical flu outbreaks. The 1918 influenza pandemic swept the world within months, killing an estimated 50 million people — more than any other illness in recorded history for the short time frame involved. One-fifth of the world's population was infected, and it struck more than 25 percent of U.S. residents.
Today, governments are more prepared, scientifically and logistically, to handle flu outbreaks. Still, there is no vaccine for swine flu, and it could take months, or more, to develop one.