Please consider the phenomenon of bacterial resistance to antibiotics.
How did the combination of human advances and basic evolutionary principles lead to this situation?
How is it a threat to humans and how might we overcome it?
Bacteria are microscopic life forms that are unicellular (and sometimes multicellular) that have their own kingdom in the taxonomic classification system. Bacteria and viruses constitute the bulk of all life forms and biomass on Earth. Some of these microbes cause disease in plants and animals, including of course humans. There are several means to kill bacteria that have been used by humans for centuries, including heat (cooking food) and harsh chemicals (using bleach to clean a surface). But killing bacteria in the human body obviously requires a different method.
Antibiotics are a class of chemical small molecule compounds (both from natural and synthetic sources) that either kill or inhibit the growth of certain kinds of microbes. Antibiotics from nature have been used for centuries by humans to treat certain conditions (various plant leaves and roots or mushrooms, etc.). The natural antibiotics in these plants and fungi were ingested by people or used topically to kill bacteria before anybody even knew that microbes existed. But in the early 20th century, scientists found ways to culture and isolate the specific antibacterial compounds from these natural sources and produce them on a commercial scale. Prontosil was the first commercial antibiotic, but penicillin during WWII was really the breakthrough drug. We now had a portable and reliable way to kill many bacterial diseases around the world.
There are two major causes for the phenomenon of bacterial resistance. One cause is something that we really can't do anything about: bacterial evolution. Bacteria are amazingly adept at surviving. They have been found in some of the harshest places on Earth. As bacterial populations have experienced changes with regard the introduction of large scale antibiotics, they have evolved and made changes to survive accordingly. The other major cause is directly our fault: overuse. Humans have grossly overused antibiotics over the decades believing that they are a miracle cure to most anything. Antibiotic resistance happens naturally as a result of bacterial evolution, but the overuse of antibiotics have lead to a quickening of the pace of antibiotic resistance. New strains of resistant bacteria have shown up that are either unaffected by classic drugs or require much larger doses of them to be effective.
The biggest threat to humans in terms of antibacterial resistance is the emergence of "superbugs", or bacterial strains that cannot be killed by common antibiotics. This requires new and more sophisticated antibiotic molecules to be developed, and these new treatments are more complicated to discover and manufacture, thus meaning that they are very expensive in comparison to classic antibiotics like penicillin. The best thing that we can do to try to overcome the problem of bacterial resistance is to curb our usage of antibiotics. This will slow down the bacterial evolutionary time line.