Actually, while it IS one of the oldest of all the treatments for infections, it WAS one of the most reliable. The problem is, many bacterial infections, due to overuse of antibiotics, have become antibiotic resistant so stronger, and stronger antibiotics, ones that even start making humans ill, had to be created. Funny thing is, some of those that were once resistant to the drug, lost that resistance because it fell out of use except for SOME infections. In adapting to other, stronger, drugs, the infections became susceptible to penicillin again. Though there are those who have allergies to molds and spores that end up having a bad reaction to it.