Alexander Flemming accidentally discovered and isolated penicillin in September of 1928. This marked the start of modern antibiotics—everything before that were just hunches and rumors that molds and penicillin could inhibit bacteria growth. This hard evidence was great news for the sick and bacteria-ridden masses, but Flemming also saw something else interesting.
He noticed that the bacteria quickly developed antibiotic resistance when too little was used or it wasn't used long enough. Though he was revered for his discovery of penicillin, he often spoke out in his speeches against using it unless extremely necessary. And when it had to be used, he insisted that patients must not skimp on it or stop before the infection is gone, or the bacteria could grow to resist even the strongest stuff!