Smallpox is caused by a virus. Specifically, it is caused by the variola virus, which is part of the orthopoxvirus family. Smallpox was a serious, contagious disease that caused high fever and a characteristic rash with pus-filled blisters. It had a high mortality rate and often left survivors with severe scarring. However, due to widespread vaccination efforts, smallpox has been eradicated globally since the late 1970s. Tuberculosis, on the other hand, is caused by bacteria, while malaria is caused by a parasite, and cholera is caused by bacteria. Therefore, smallpox is the only disease in the list that is caused by a virus.