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Deadly diseasesBacteria and viruses that are deadly to one type of creature can evolve quickly to infect another. While the swine flu outbreak is the latest example, a host of infectious and deadly diseases have hopped from animals to humans and from humans to animals.
The cross-species infection can originate on farms or markets, where conditions foster mixing of pathogens, giving them opportunities to swap genes and gear up to kill previously foreign hosts (i.e. you). Or the transfer can occur from such seemingly benign activities as letting a performance monkey on some Indonesian street corner climb on your head. 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. There are more than three dozen we can catch directly through touch and more than four dozen that result from bites.
But disease-carrying parasites are not picky about hosts. Human diseases can decimate animal populations, too, from such well-meaning activities as ecotourism.
SOURCE: CDC; WHO; National Archives; LiveScience reporting
Influenza pandemicsSlide 2 of 21
Influenza pandemicsThe swine flu outbreaks cropping up in several countries now are nothing — so far — compared to historical flu outbreaks. But with more people on the planet, and more of them huddled in cities and more of them traveling so easily, the potential for pandemic is not lost on health officials.
Flu history is frightening: 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. Unlike some flu strains that mainly kill the elderly, children, and those with compromised immune systems, the 1918 strain hit young adults hard. In one year, the average life expectancy in the United States dropped by 12 years.
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.Slide 3 of 21
Bubonic plagueSlide 4 of 21
Bubonic plagueNothing 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. It is the epitome of plague. 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. Symptoms include fever, chills, weakness, and swollen and painful lymph nodes. Even today, if not treated, death ensues.
The plague of the 14th-century resulted after the rare bacteria had been dormant for centuries in Asia's Gobi desert. After awaking in the 1320s, it piggybacked along trade routes from China, through the rest of Asia and eventually to Italy in 1347, then later to Russia.
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.Slide 5 of 21
Diseases that biteSlide 6 of 21
Diseases that biteA range of zoonotic diseases — thought to be on the rise — are caused by animal bites that kill hundreds of thousands of people every year. Mosquitoes lead the way: Malaria infects 350 million or more people every year, and more than 1 million die, most of them young children in Africa south of the Sahara. Mosquito-borne dengue fever infects some 50 million people annually; about 500,000 are hospitalized and about 2.5 percent of those die.
The problem is growing, with insect-borne disease outbreaks becoming more common and more virulent, and epidemics having spread to the Americas. Scientists say the warming climate will only make matters worse.
Illustrating our illness connection to animals and especially pets, rabies kills about 55,000 people globally each year, mostly in Asia and Africa. Most deaths follow a bite from an infected pet dog, though wild animals can carry rabies too. And an estimated 16 million or more people from Mexico to Argentina are affected by Chagas disease, a chronic, frequently fatal infection transmitted by the feces of blood-feeding bugs called triatomines (commonly called "kissing bugs"). Chagas is often spread by dogs or even chickens that are kept indoors at night, giving the bugs access to the people.
You don't even have to be bitten by bugs or animals to get some deadly diseases from them. Hantavirus is carried mostly by deer mice. You can catch it by breathing dust contaminated with mouse droppings. Lack of appetite, fever, vomiting and muscle aches characterize Phase 1. Phase 1? Yep. Just when you start to feel better, you may get a stiff neck, another fever, confusion and have trouble moving. It is incurable, but most people recover. About 1 percent die.Slide 7 of 21
HIV/AIDSSlide 8 of 21