Many drugs and other medical treatments have worked at first, then gradually become ineffective. The reason is that disease-causing agents such as bacteria and viruses (collectively called pathogens) gradually evolve resistance to the drug. Evolve is precisely the right word, because the appearance of drug resistance is an evolutionary process in which the drug itself is the agent of selection for resistant pathogens that are already present in the population of pathogens. (See How Scientists Know About Evolution.)
Resistance is a property or trait of the pathogen, not of the patient or the drug. A common misconception is that the patient becomes resistant, and that the drug fails because it changes the patient in some way that makes the drug ineffective. This would mean that the drug would begin to fail in individual patients, but only in those who have had the drug before. But as observed with resistant pathogens, the drug fails for all patients who become infected with that pathogen, even if the patient has never taken the drug before.
The following videos will show how resistance develops, and approaches to overcoming or avoiding pathogen resistance.
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(not finding really good videos on this subject, but still looking -- if you know of good ones, please suggest -- see my contact information on this page)
