Bacteria and viruses that are deadly to one type of creature can evolve quickly to infect another. While the <a href="http://www.livescience.com/topic/flu">swine flu outbreak</a> is the latest example, a host of infectious and deadly diseases have hopped from animals to humans <i>and</i> from humans to animals.
<p> 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, <a href="http://www.livescience.com/health/090428-swine-flu-viral-evolution.html">do some viral dancing</a>, and evolve to morph you into a deadly, contagious host.
<p> 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.
<p>But disease-carrying parasites are not picky about hosts. Human diseases can decimate animal populations, too, from such well-meaning activities as ecotourism.
<p> <i>SOURCE: CDC; WHO; National Archives; LiveScience reporting</i>
The <a href="http://www.livescience.com/topic/flu">swine flu outbreaks</a> 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.
<p> 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.
<p> 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.
<p> Today, governments are more prepared, scientifically and logistically, to handle flu outbreaks. Still, <a href="http://www.livescience.com/health/090429-flu-vaccine.html">there is no vaccine</a> for swine flu, and it could take months, or more, to develop one.
Nothing beats the 14-century <a href="http://www.livescience.com/history/080428-hs-black-death.html">Black Death</a> (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.
<p>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.
<p>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.
<p>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.
Diseases that bite
A range of zoonotic diseases — <a href="http://www.livescience.com/health/061108_zoonotic_diseases.html">thought to be on the rise</a> — 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.
<p>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.
<p>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.
<p>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.
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. It destroys the immune system, opening the door to a host of deadly infections or cancers. Example: Tuberculosis (TB) kills nearly a quarter of a million people living with HIV each year.
<p>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.
<p>Recently, researchers found that HIV was in the United States <a href="http://www.livescience.com/common/media/video/player.php?videoRef=080207-aidsheresooner">as early as 1969</a>, much earlier than had been thought.
Insane mind parasite
The bizarre parasite <i><a href="http://www.livescience.com/strangenews/060803_tgondii_culture.html">Toxoplasma gondii</a></i> 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.
<p>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).
<p>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 <i>T. gondii</i> infection also have higher average neuroticism scores, one study found.
A large cat dining on the entrails of one our early ancestors thousands of years ago <a href="http://www.livescience.com/animals/060628_cats_ulcers.html">contracted an ulcer-causing bacteria</a>, <i> Helicobacter pylori</i>, that spread to lions, cheetahs and tigers, scientists figure. It persists to this day in large cats.
Ebola is a widespread threat to gorillas and chimps in Central Africa, and may have spread to humans from people who ate infected animals. It is now transmittable human-to-human, by contact with the blood or body fluids of an infected person, and it has killed a few hundred people in each of several outbreaks going back to the mid-1970s.
<p>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.
<p>Ebola may be carried by bats, researchers think, because when bats are infected with it, they don't die.
Polio, Yaws, Anthrax
Studying wild animal populations can be difficult. But scientists have speculated that chimps at Gombe Stream National Park in Tanzania contracted polio from humans, according to Fabian Leendertz, a wildlife epidemiologist at the Robert Koch-Institute and Max Planck Institute for Evolutionary Anthropology in Germany.
<p>There have also been concerns that gorillas contracted yaws, a disease related to syphilis that is not sexually transmitted, from humans, Leendertz added.
<p>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.
Human virus that kills chimps
Ecotourism fuels outbreaks of respiratory diseases among African chimps. The human respiratory syncytial virus (HRSV) and human metapneumovirus (HMPV) kill infant humans in developing countries. Nearly all humans have had contact with these germs, though, and so have developed antibodies naturally designed to fight them. But in the first confirmed evidence of viruses transmitted directly from humans to wild great apes, the virus <a href="http://www.livescience.com/animals/080129-ape-viruses.html">killed entire populations of chimps</a> in part of West Africa between 1999 and 2006.
Gorillas gave humans 'crabs'
Humans <a href="http://www.livescience.com/health/070307_gorilla_lice.html">caught pubic lice from gorillas</a> 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).