Bacteria are all around us. They outnumber us vastly, and fortunately, nature has created a balance such that we seem to be able to coexist with these tiny organisms (for the most part). However, there are some disease-causing bacteria that are pretty nasty and deadly, so we were pretty lucky to discover antibiotics like penicillin. But the process of evolution is rendering our antibiotic medicines useless against a growing population of bacteria. Here are just a few informative links on these superbugs.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post