AIDS, ebola virus, influenza and SARS - these are just some of the infectious animal diseases that have jumped the species barrier to humans. Some are fatal, others are not. Severe acute respiratory syndrome, which startled health workers with its rapid spread around the world, is but the latest example of an animal disease to infect humans.
Researchers say SARS appears to have sprung from a virus mostly infecting masked palm civets, a cat-like tree dwelling animal that is eaten as a delicacy in China. Dr. William Schaffner is chairman of the Department of Preventive Medicine at Vanderbilt University School of Medicine in Nashville, Tennessee. He explains how the animal virus can transfer to humans. "The infectious agent undergoes a mutation, a genetic change, that now makes it able for this organism, the virus or whatever, to actually infect humans, which it could not before," he says.
Animal viruses and bacteria have long infected humans, according to Christopher Wills, who teaches biological sciences at the University of California in San Diego. The first documented case of a disease suspected of being transmitted by animals, occurred in Athens in the sixth-century BC.
During the Middle Ages, Professor Wills said, one-third of Europe was wiped out by two massive outbreaks of bubonic plague, also known as the Black Death. The lethal illness was transmitted to humans by diseased rats. "The one that everyone knows about, the Black Death, happened in the 14th century," he says. "These two plagues, however, were not the only outbreaks of the Black Death. The Black Death outbreaks happened over and over again. In fact, the Black Death broke out in India and in China at the beginning of the 20th century several times, killing millions of people."
In the past 50 years, previously unknown diseases have appeared with alarming frequency. Vanderbilt University's Professor Schaffner says there are a number of reasons why. "We are being more clever at being able to detect them. The second is, we as humans are going into the habitat of the animals. So, we are coming into closer contact with the animal, making it easier for the transmission to occur," he says.
Disease transmission is also fueled by the free movement of people and animals around the world. By the time the SARS epidemic was brought under control in early July, the disease had spread to nearly 30 countries and infected around 8,000 people, 800 of whom died.
Once inside humans, animal viruses or bacteria may continue to mutate or change their appearance, making it difficult for the body's natural defenses to recognize and fight off the infection. That is when an animal disease may become lethal to humans. HIV is one such example, it is a monkey virus that crossed the species barrier.
But not all animal-borne diseases are deadly to humans. David Heymann heads the World Health Organization's polio eradication effort, and most recently coordinated the international response to SARS.
During the past 25 years, Dr. Heymann notes, there have been more than 30 newly-identified diseases that have spread to humans, either through the bite of an infected insect or by direct contact, such as the handling or eating of animals. "In most of these infections, they get into humans, they cause disease, and they do not seem to transmit easily from human to human, the exception being, of course, influenza. And that is our great fear, that we will have shortly a major epidemic of a new influenza, which will cause high mortality, and for which we are not prepared," he says.
His fears are not unfounded. The Spanish Flu epidemic of 1918, caused by a virus spread to humans by chickens, along with avian flu strains in the latter part of the 20th century killed at least 20 million people before they ended.
This report is part of VOA's series on world health.