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
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.
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.
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.
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.
Recently, researchers found that HIV was in the United States as early as 1969, much earlier than had been thought.
However, its primary host is house cats, in which the microbe reproduces sexually. Cats left to roam are more prone to picking it up. You can get it from cat feces. The bug is also found in many other mammals, too (where it reproduces asexually).
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.
The awful symptoms: sudden onset of fever, intense weakness, muscle pain, headache and sore throat, often followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, both internal and external bleeding. It's deadly somewhere between 25 and 90 percent of the time depending on the strain.
Ebola may be carried by bats, researchers think, because when bats are infected with it, they don't die.
There have also been concerns that gorillas contracted yaws, a disease related to syphilis that is not sexually transmitted, from humans, Leendertz added.
Gorillas and chimpanzees in West Africa have been killed by outbreaks of anthrax, which might have originated from cattle herded by humans, although Leendertz said these events may have been caused by anthrax existing naturally in the forests.