In the before times, there was the influenza pandemic of 1918. It killed more people than World War I, between 20 and 40 million. The virus ravaged the globe with high infection and mortality rates, and science could do little to stop it. Eventually the pandemic died out, but not before the damage was done.
At the time, immunization was primitive and little could be done about viruses. Bacterial infections like syphilis and tuberculosis were untreatable and often resulted in death.
Then, in 1928, a breakthrough in medicine occurred. British scientist Alexander Fleming found a naturally growing mold attacking and killing Staphylococcus. Penicillin was discovered, and it became the first antibiotic. In the 1940s, penicillin became widely available and was used to treat soldiers during WWII.
Today antibiotics are used for a multitude of conditions, and this may be their downfall. There are both broad-spectrum antibiotics, which kill a variety of types of bacteria, and narrow-spectrum antibiotics, which kill specific bacteria. At least 150 million antibiotic prescriptions are written in the U.S. each year, according to Healthy Children. Their use, and overuse, is contributing to the development of antibiotic resistant strains of bacteria. If this trend continues, we could be thrown back into the pre-penicillin era.
Healthy Children History of Antibiotics: http://www.healthychildren.org/English/health-issues/conditions/treatments/pages/The-History-of-Antibiotics.aspx?nfstatus=401&nftoken=00000000-0000-0000-0000-000000000000&nfstatusdescription=ERROR%3a+No+local+token
Stanford University The Influenza Pandemic of 1918: http://virus.stanford.edu/uda/