I jest, sort of, but this much is true: every time you take antibiotics, billions of domesticated gut flora die. As I mentioned last week, antibiotics are designed not to target human cells, but the same cannot be said for the commensal bacteria living in our guts. See, most antibiotics don't discriminate between "good" and "bad" bacteria. They target bacteria. They aren't us, they are foreign entities, but we wouldn't be us without them. We need them to function properly. It's a bit like bringing in an exterminator to kill the bugs infesting your house, and the guy ends up killing your dog and making your cat act funny, along with killing the insects. The job is done, and he technically did what you requested, but now you have to tell your kid that Buddy moved to a farm upstate to go be a sheepdog and figure out how to deal with your cat peeing on the sofa and scratching up your stomach (leaky gut, get it?). Not very fun, and not what you bargained for.
The results of a 2010 study on the lasting effects of antibiotics on one's gut flora are rather scary. Over a 10 month period, three individuals – humans – each went on two courses of ciprofloxacin, an extremely commonly prescribed antibiotic often used to treat bone and joint infections, respiratory tract infections, gastroenteritis, endocarditis, urinary tract infections, cellulitis, infectious diarrhea, anthrax infection, typhoid fever, and skin infections to name more than a few. In other words, it's regarded as a trusty all-purpose antibiotic, effective across all species (vets often prescribe cipro). So, what happened to the patients' gut flora populations after taking cipro?