Measles Is Considered the Most Contagious Virus in the World, Posing a Major Challenge for Public Health Agencies

The United States is facing an unprecedented measles outbreak, with 2,813 cases reported as of August 21 — the highest number of infections since the 1990s. The disease, which was declared eliminated in the country in 2000, has resurged amid declining vaccination rates and vaccine skepticism within the Trump administration and parts of the American public. The current tally already surpasses the annual total for 2025, which saw 2,286 cases, and is nearly 10 times higher than the 282 cases recorded in 2024. Health officials warn that the start of the school year is likely to further facilitate transmission of the virus. Measles is considered the most contagious virus in the world, posing a major challenge for public health agencies.

"One person can infect between 15 and 20 others if they are not vaccinated, said epidemiologist Antoine Flahault, compared to just one or two for the flu. "The flip side of this very high contagiousness is that it requires an extremely high vaccination rate, exceeding 95% of the population, added the Paris Cité University professor of public health. The outbreak highlights the consequences of falling immunization coverage in the country.