Many infectious diseases that killed by the millions were greatly reduced in the 20th century, with one notable achievement being the eradication of smallpox, and considerable progress being made toward the eradication of polio (polio eradication being expected to be completed within the first decade of the 21st century) and guinea worm disease (expected to be eliminated everywhere by 2005).

Other diseases, such as diphtheria, typhoid fever, tuberculosis and whooping cough were greatly reduced throughout the world due to childhood immunisation programs, improved sanitation, and the use of antibiotics. Malaria, even though easily treatable, is still a major killer in poor countries.

In the United States the death rate from pneumonia and influenza fell 93% in the 20th century; bronchitis was once responsible for 3% of deaths in America - that figure has fallen to nearly one-tenth of 1%.

Two major pandemics occurred in the 20th century: an outbreak of a severe strain of influenza (the "Spanish Flu") which killed some 25 million or more people in 1918-1919, and the appearance of AIDS in the 1980s on. AIDS is transmitted by a virus, and viral diseases can usually only be overcome by vaccination. An effective AIDS vaccine has eluded researchers so far. Anti-viral drugs have been developed, but they are too expensive for most people suffering from AIDS.

A concern is the appearance of antibiotic-resistant strains of infectious organisms. It is believed that the overuse of antibiotics, including their use in animal husbandry, contributes to this development.