Their Bugs Are Worse Than Their Bite

Zoonoses

Almost by definition, “novel” viruses tend to come from other species.⁷²¹ In 1959, the World Health Organization defined the term “zoonosis” to describe this phenomenon,⁷²² from the Greek zoion for “animal” and nosos for “disease.” Most emerging infections are RNA viruses such as Ebola, HIV, or influenza⁷²³ —not surprising, given their ability to mutate rapidly, evolve, and adapt to new hosts.⁷²⁴ Although many doctors of today learned in their medical school textbooks that viruses were species-specific and therefore couldn’t jump from animals to people,⁷²⁵ we now know that viruses are, as the Mayo Clinic describes, “masters of interspecies navigation.”⁷²⁶

The exact proportion of emerging human diseases that have arisen from (other) animals is unknown. In 2004, the director of the CDC noted that “11 of the last 12 emerging infectious diseases that we’re aware of in the world, that have had human health consequences, have probably arisen from animal sources.”⁷²⁷ An editorial in Lancet published the same year insisted that “[a]ll human diseases to emerge in the past 20 years have had an animal source….”⁷²⁸ In any case, experts agree that it’s a sizeable majority.⁷²⁹ Eleven out of the top 12 most dangerous bioterrorism agents are zoonotic pathogens as well.⁷³⁰ The Institute of Medicine published a report on the factors implicated in the emergence of disease in the United States. “The significance of zoonoses in the emergence of human infections,” it concluded, “cannot be overstated.”⁷³¹

According to the World Health Organization, the increasing numbers of animal viruses jumping to humans is expected to continue.⁷³² The zoonotic virus pool is by no means exhausted.⁷³³ “If you look at the animal kingdom—from goats, sheep, camels, poultry, all fish, just about any animal you can name—they [each] have probably 30 or 40 major diseases,” notes the WHO expert who led the fight against SARS. “So the possibility for exposure is huge.”⁷³⁴ Estimates as to the number of zoonotic diseases run into the thousands.⁷³⁵ “For every virus that we know about, there are hundreds that we don’t know anything about,” said one professor of tropical medicine at Tulane who studies emerging viruses in Africa. “Most of them,” he said, “we probably don’t even know that they’re out there.”⁷³⁶

Transmissions of disease from animal to person are not new.⁷³⁷ Most of the human infectious diseases that exist today originally came from animals.⁷³⁸ Many of humanity’s greatest scourges—including influenza—can be traced back thousands of years to the domestication of animals.⁷³⁹