The Microbial Threshold for Illness: Unveiling the Magic Number

The number of microbes needed to make us sick varies by pathogen and is known as the infectious dose. Some pathogens, like norovirus, require a very small number of organisms to start an infection, making them highly transmissible. Researchers determine infectious doses through human challenge studies, animal models, and observational studies. The infectious dose for the virus that causes Covid-19 is estimated to be around 100 to 400 plaque-forming units (PFUs) on average. Vaccination can raise the infectious dose by activating the immune system's defenses. To reduce the risk of infection, it is recommended to practice social distancing, wear masks, improve ventilation, and get vaccinated.