Deadly outbreaks and novel diseases have challenged human existence throughout history, profoundly impacting economics, culture, and commerce, killing world leaders and bringing down empires, says David Morens, a zoonotic disease expert at the National Institute of Allergy and Infectious Diseases. Many of the viruses and bacteria behind these outbreaks existed for millennia without causing widespread harm. Human behavior has changed that. “ Few people realize that measles, plague, and other diseases go back thousands of years, with Neolithic origins, ” he says.

The growing human population, increasing globalization, and environmental damage are all accelerating the process, says William Karesh, an executive vice president at EcoHealth Alliance, a New York-based nonprofit that studies zoonoses, or diseases that spread between animals and humans. “ The laws of biology haven ’ t changed, but the playing field has changed dramatically, ” he says.

The result: dangerous new human diseases are emerging at unprecedented rates, including Marburg virus, avian flu, AIDS, severe acute respiratory syndrome ( SARS ), Nipah virus, swine flu, Ebola, Lyme disease, chikungunya, Zika, dengue, Lassa fever, yellow fever, and now COVID- 19. Some 2.5 billion people are infected with zoonotic diseases each year, and because many of these ailments have no cure, they kill about 2.7 million annually, according to the U. S. Centers for Disease Control and Prevention.

Unlike previous centuries when diseases took time to spread, the infected can now board a plane and disseminate their germs worldwide before they even show symptoms. COVID- 19 emerged in China just 21 months ago, and cases have since been reported in 223 countries and territories. Humans have also enabled disease-carrying ticks and mosquitoes to expand their ranges by altering the climate. As the planet warms, these insects move into new territory.