An abrupt change in C.D.C. guidance underscores a little-known phenomenon: On occasion, monkeypox can be transmitted through aerosols, similar to the coronavirus.
As the number of cases outside Africa approaches 800, governments are scrambling for a limited pool of vaccines and treatments with unclear effectiveness.
Older people who received smallpox vaccinations may yet have some immunity, researchers say. Healthy children and adults generally do not become severely ill.