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.