“For every death, there’s about 21 other infections,” she said modeling showed. So most of Philadelphia’s 50,000 people probably were infected. “They were either lucky that they didn’t develop disease or had mild disease compared to those that were unlucky and died.”Yellow fever is one of more than 200 known human viruses, according to the National Institutes of Health. They include those that cause HIV/AIDS, Ebola, polio, smallpox, measles, mumps, rabies, the common cold and now the novel coronavirus.