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 evolved from rinderpest virus, 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. Middle East respiratory syndrome coronavirus (MERS-CoV), a virus found circulating in the Arabian Peninsula in 2014, was probably first passed to people by camels, though it may have originated in bats. 

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 originated in animals. The next deadly pandemic to sweep the world could very likely jump species in this way.

 

Direct Transmission

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 encounters often spurred by ecological change, the pathogen moves from the natural host to humans. Severe acute respiratory syndrome (SARS) is a recent example of this. In spring 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 virus originated in Chinese horseshoe bats, animals used for food and medicine in many parts of Asia and traded in wildlife markets. There the virus infected both small carnivores called civets as well as the people who worked at the markets where they were sold and in restaurants where these animals were served as food. The virus infected 8,098 people, 774 of whom died—nearly a 10 percent mortality rate. Fortunately, no human infections have been found since early 2004.

 

There are many other examples of direct transmission from animals to humans. Toxoplasmosis, a parasitic disease that can cause mild flu-like symptoms in humans (but potentially more serious illness in developing fetuses and individuals with a compromised immune system), infects many warm-blooded animals. Cats are essential for this parasite to spread because 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. Nipah virus, which can cause fatal encephalitis (inflammation of the brain), emerged in 1998 in Malaysia. This virus is carried by fruit bats, but it moved into pigs and then spread to pig farmers and slaughterhouse workers who were exposed. 

 

Vector-Borne Diseases

Diseases transmitted to humans indirectly via an insect or an arthropod (animals with jointed appendages and exoskeletons, such as ticks) are called vector-borne diseases. Vectors carry disease-causing viruses, bacteria, or parasites from one host to another, delivering these pathogens to humans and other warm-blooded 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 affecting New Yorkers. Seven people died and 62 were hospitalized. Until then the virus had been confined to Africa, Europe, the Middle East, and West Asia. 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 also spread geographically, moving rapidly across North America and into Latin America. According to 2015 data, there were 1,996 reported cases of West Nile virus in the United States, with 111 deaths. 

 

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, again showing the unintended consequences that global trade and travel can have on the spread of infectious disease.

 

Dengue fever, also known as break-bone fever, is a mosquito-borne virus that results in flu-like symptoms. It was detected in Key West, Florida, in 2009 and by the end of 2010 there were 21 cases reported in Key West, prompting concern that the virus had emerged in the United States and could become endemic. 

 

Chikungunya, a rarely fatal virus also transmitted by mosquitoes, crossed the Atlantic Ocean in 2013. Since then it has been detected in more than 10 countries in the Americas and has resulted in more than 780,000 suspected cases, with 15,000 being confirmed.

 

Zika, originally identified in Africa more than 60 years ago, was first recognized in the Americas in Brazil in 2015. It has since spread to almost 50 countries, including the United States.

 

Many other infections, including malaria, Lyme disease, yellow fever, and typhus, are spread to humans from animals (including from other humans), via the bites of insects and other arthropods.