When it comes to spreading viral and bacterial infections, some people are more contagious than others—much more contagious.

Known as superspreaders, they amount to roughly 20 percent of the population, but they account for transmission of about 80% of certain infectious diseases, scientists estimate. The phenomenon has been observed, among other contagions, during the global SARS outbreak in 2002 and 2003 and as far back as Typhoid Mary, a cook in New York who infected dozens of people with typhoid fever in the early 1900s without falling ill herself.

Now, scientists are working to find the reasons why some people spread disease more than others. Of course, circumstances play a role, such as a crowded day-care center, and some pathogens are naturally more virulent than others. Recent experiments suggest the body’s immune system at times also may play a role—not just to protect against infection but also to spread more of the pathogens to others.

It isn’t clear which is the bigger contributor to being a superspreader—people’s behavior or their immune-system response. But some scientists say both factors seem to play a role.

“This is a phenomenon that’s been observed but nobody really fully understands it,” said Denise Monack, an associate professor in the microbiology and immunology department at Stanford University. Ultimately, researchers would like to find a way to predict who is a superspreader to help contain the spread of infectious diseases, she said.

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