Infectious dose varies by pathogen, affecting transmissibility
Pathogens must overcome the body's defenses to make us sick. The number of microbes needed to cause illness varies by pathogen, known as the "infectious dose." Norovirus, for example, has an incredibly low infectious dose of 18 individual viruses. Researchers determine infectious doses through human challenge studies, animal models, and observational studies. The virus that causes Covid has an estimated infectious dose of 100 to 400 plaque-forming units, making it highly transmissible.