The discovery of penicillin has been long lauded as the perfect accident. Though researchers decades before had noticed that a certain mold had antibacterial properties, Alexander Fleming took the discovery credit for deciding his contaminated culture was worth studying in 1928. Purified penicillin could repress Gram-positive bacteria to a masterful degree, with few side effects, and it quickly became the popular choice to stave off infections or treat infectious diseases like Scarlet fever. 
Scarlet fever used to ravage young children with rashes and deadly fevers, making appearances as far back as the 1500s. Contracting it was often fatal, and if the child survived, she could have lingering heart and kidney problems for the rest of her life. The disease is caused by the Gram-positive bacterium Streptococcus pyogenes — which, normally benign, is transformed into a raging, poisonous monster by the virus T12 — and could spread easily through the air. 
We don’t hear much about scarlet fever anymore, thanks to penicillin. We also don’t hear about the treatments that preceded our miracle mold. My favorite has been rendered more a test of maturity than anything else. It was called the Dick Test.
Pioneered in 1924 by medical doctors and married couple George F. Dick and Gladys Henry Dick, the Dick test helped identify children who would be susceptible to Scarlet fever. To create this test, the Dicks isolated and sterilized the guilty streptococcus bacterial strain from an active case of scarlet fever. They then injected the distillation into ten volunteers, just below the skin. Two of the volunteers developed Scarlet fever, which the Dicks took to be a sign that the remaining eight had adequate immune systems. 
They later toned the injections down by diluting their filtrated substance so that susceptible patients wouldn’t develop full-blown Scarlet fever — just a pleasantly red skin reaction around the area of injection. This mild infection acted as a vaccine to prime the volunteer’s immune system, the same way Pasteur had used relatively benign cowpox to prevent the more lethal smallpox. The Dicks found that re-injecting patients who initially showed a positive skin reaction would make their small skin reaction vanish. Patients were considered immunized and cured. 
Then penicillin came along and wiped out the need for vaccinating against Scarlet fever at all. But for a brief period of time between the 1920s and 1940s, being a Dick to children helped save lives.