Penicillin: Working Mechanism and Triumph in History of Medicine
Penicillin is an anti-bacterial drug. Bacteria are tiny one-celled organisms. Many kinds are harmless (or even beneficial), but some kinds cause serious diseases. Penicillin's working mechanism is complicated, but basically, it fights diseases by getting into the bacteria and stopping them from building new cell walls. After that, either the bacteria break down on their own (some kinds need to constantly rebuild their walls or die), or your body's immune system has a much easier time killing them.
Taking penicillin (or any other anti-bacterial drug) is pretty indiscriminate, so it also kills off harmless bacteria and beneficial bacteria in your digestive tract. Also, anti-bacteria do nothing against diseases caused by viruses. Penicillin was the first drug of its kind, which meant that when it was new, it was the only way to cure many kinds of infections. Some of those infections had been considered death sentences until then, so penicillin was hailed as a life-saver. This was especially true during World War II when penicillin was first used in a big way. Only the US and UK had it, and it saved thousands of lives that would otherwise have been lost to infected battle wounds. Penicillin also cured other diseases that are less lethal but used to carry a huge social stigma. Syphilis causes blindness in the children born to infected women, and will (after many years) eventually drive you insane (and then kill you). Also, syphilis is spread by sexual contact, so being infected carried a huge stigma -- some states required blood tests before you could get married, because of the risk of blind children. Being cured of syphilis by penicillin meant you could still get married and have a family, instead of being a social outcast.
Another big disease changed by penicillin was tuberculosis (TB). TB is a crippling lung infection that will kill you eventually and is highly contagious (transmitted by coughing). Poor people who caught it sometimes wound up infecting their whole family. Richer people who caught it would be sent to live in isolation at a sanitarium, which was like a cross between a resort and a hospital. Penicillin absolutely cured people of TB and gave them their lives back.
Both TB and Syphilis still exist today, though. They weren't wiped out by penicillin, and neither were any other infectious diseases. In fact, while penicillin worked like a miracle for a while, the bacteria that survived it were ones that were less susceptible to it. They reproduced, and eventually, all the bacteria left were penicillin-resistant. Penicillin gradually stopped working, and today is almost entirely useless. That's evolution at work -- the bacteria evolved to survive in a new environment, one that had penicillin. When that happened, doctors had to invent new drugs to replace penicillin. These worked in other ways, attacking other aspects of the bacteria, and the bacteria were again caught defenseless. But again, the bacteria that did survive were the resistant ones, and the diseases evolved.
Every antibiotic drug 'trains' the bacteria to survive it, and becomes useless in time. Today, antibiotic resistance is a serious problem in medicine. Some bacterial infections, like MRSA, get into hospitals and infect patients who are in hospital for other reasons. I'd strongly recommend against calling penicillin 'the single greatest medical achievement of all time'. For one thing, it's a needless superlative: lots of things are really great, and none are perfect. For another, it won't hold up by objective measures, like most lives saved. The eradication of smallpox is considered a biggie among medical advancements.
Smallpox was a huge and brutal killer, and vaccines eradicated the entire disease -- it no longer exists as a disease. Another big vaccine triumph was polio. The vaccine for polio was hailed at the time it was invented because polio was a big killer/crippler of small children. Today, polio is almost eradicated, but it still hangs on in a few places. If you wanted to point to the single invention that did the most to reduce harm and suffering from disease, it's got to be indoor plumbing. Toilets that lead to sewer systems, and clean drinking water right from the faucet are great conveniences and obviously aren't medical inventions. But they've saved more lives than medicine ever has.
