evidence: Herd immunity can't be reached for every disease, but measles is one example of the idea. Measles is a disease caused by a virus that spreads quickly among people who've never had the disease or the measles vaccine. [...] People who are at high risk of serious illness, or who are regularly around people at high risk, may choose to wear a mask. If you came in contact with a germ, are sick or are getting over sickness, wearing a mask can help protect the people around you. [...] - Preventing spread of respiratory viruses when you're sick. Centers for Disease Control and Prevention. https://www.cdc.gov/respiratory-viruses/prevention/precautions-when-sick.html. Accessed May 14, 2024.