You may have a virus, like the flu, which means there would be no point in giving you penicillin. The reason is that penicillin is an antibiotic. Antibiotics have no effect on viruses; they only destroy bacteria, which are living cells. Antibiotics affect bacteria by preventing the creation of nucleotides in the bacteria, which stops them from reproducing. They don't harm human cells, which is why they're so effective at knocking out bacterial infections. Penicillin was one of the first antibiotics. Like all antibiotics, various forms of penicillin work only on certain bacteria, and these bacteria can develop resistance to the drugs over time.
The reason bacteria build up resistance is that bacteria reproduce very quickly. Their rapid reproduction makes them likely to have mutations. Even though a drug may be effective against millions of bacteria, it only takes one cell to mutate to create immunity to a certain antibiotic. That mutation then multiplies rapidly and can spread to other people, making the antibiotic ineffective against the disease.
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