But COVID-19 is also a different epidemiological beast than smallpox and polio—two diseases the US has eliminated through mass vaccination. The two historical viruses are ancient, so people had been living with them for centuries before the vaccines came around. Long before Washington inoculated his troops against smallpox, people in Africa, the Middle East, and elsewhere were applying the same method to survive the disease, Rogers explains. Similarly, babies may have gained immunity to polio from exposure to small amounts of the virus in contaminated water. COVID-19, on the other hand, is a novel disease derived from a more lasting class of viruses. Humans have little to no immunity to it, and vaccination is crucial to stopping its spread.