Self-medicating irresponsibly?

It's the fact (not speculation, but observed directly, many times over) that pathogenic organisms with their great number of individuals, their high reproduction rates and, in some cases, ability to exchange genetic information (see bacterial conjugation) will quickly develop immunity towards a given drug. This will happen no matter what, but abusing the drug by administering in cases were it isn't really necessary accelerates the process immensely.

It's easy to see why: The more people you give the drug, the more of the pathogenic organism is exposed to it which every time is an opportunity for it to develop immunity. Limit its being administered to only those who cannot be treated otherwise, and you can get a few more months, or years before it gets useless.

See MRSA (Methycillin Resistant Staphylococcus aureus) for an example where a bacterium is creating massive problems because there are now strains around that respond to no antibiotic at all. People infected with it face a really bleak outlook, available treatments include excising (cutting out) infectede tissue up to amputating limbs, and sometimes even that does not help. Many die.

As an alternative to preventive administration of antibiotic/antiviral drugs, immunization through inoculation is a good idea.