Many of the diseases that afflict people today
are caused by microbes
whose ancestors came from animals first domesticated by early humans. Biologists believe that the measles
virus stemmed from canine distemper and rinderpest, an affliction of cattle; that rhinoviruses, agents of the common cold, came to us from horses; and that smallpox
is a close cousin of cowpox.
Of the 37 new infectious diseases identified in the past 30 years, more than two-thirds sprang from animals.
Infections transmitted from animals to humans are called zoonoses,
or zoonotic diseases. Of the more than 1,700 known viruses, bacteria, and other pathogens that infect people, more than half either originated in or now come directly from animals. The rest come from the environment around us, such as soil, water, and air. And of the 37 new infectious diseases identified in the past 30 years, more than two-thirds sprang from animals. The next deadly pandemic to sweep the world could very likely jump species in this way.
Some zoonotic infections move directly from animals to humans. In such cases, an animal is the natural host
—or reservoir—for the pathogen, and through an evolutionary
twist of fate, the pathogen moves from the natural host to humans. Severe Acute Respiratory Syndrome (SARS)
is a recent example of this. In the spring of 2003 this new and deadly viral illness swept out from China’s Guangdong Province and spread rapidly around the world before it was contained that summer. SARS originated in Chinese horseshoe bats, animals that are used for food and medicine in many parts of Asia, and was then “amplified” through the infection of civet cats, a step leading to a mutation
that makes the disease transmissible to humans. The virus infected 8,098 people, of which 774 died—a nearly 10 percent mortality rate. Fortunately, no human infections have been found since early 2004.
There are many other examples of direct transmission. Toxoplasmosis
, a parasitic disease that typically causes mild flu-like symptoms in humans (but potentially more serious illness in individuals with compromised immune systems), infects many warmblooded animals. Cats play an important role in spreading the disease when they become infected by eating infected rodents or small birds and then pass the parasite to humans through their feces. Leptospirosis
, a bacterial disease spread through the urine of infected animals, or through soil or water contaminated by infected urine, can cause a wide range of symptoms in humans, including high fever, vomiting, and even meningitis and liver failure. The Nipah virus
, which can cause fatal encephalitis (inflammation of the brain), emerged in 1998 in Malaysia. Harbored in fruit bats, the virus afflicted slaughterhouse workers who had caught it from pigs.
Diseases that are transmitted to humans indirectly via an insect, an arthropod (animals with jointed appendages and exoskeletons
, such as ticks), or another animal (such as snails, which deliver the parasite responsible for schistosomiasis
) are called vector-borne diseases.
Nearly half the world’s population is currently infected with a vector-borne disease.
Vectors carry disease-causing viruses, bacteria, or parasites from one host to another, delivering these pathogens to humans and other warmblooded hosts. The vectors themselves typically suffer no ill effects from the organisms they carry. In 1999, for example, a mosquito-borne infection—West Nile virus
—suddenly began targeting New Yorkers. Seven people died and 62 were hospitalized. Until then the virus had been confined to Africa, West Asia, and the Middle East. Today, the infection caused by West Nile virus has fully established itself in North America, flaring up in the summer and continuing into the fall. Since 1999 the virus has spread rapidly across North America and into Latin America. In 2009 there were 720 reported cases of West Nile virus in the United States, of which 32 were fatal.
Wild or domestic animals are natural reservoirs for many vector-borne diseases. The main reservoir host for West Nile virus is wild birds. The New York City strain of the virus was virtually identical to a strain taken the previous year from a dead goose in Israel. Scientists speculate that an infected mosquito, human, or bird may have brought the pathogen to this country on a plane or ship.
Many other common infections, including malaria
, yellow fever
, Lyme disease
, and typhus, are spread to humans from animals via the bites of insects and other arthropods. In fact, nearly half the world’s population is currently infected with a vector-borne disease.