its called natural selection. The meds kill off those that are susceptible while those that are not are given free reign to multiply. This is resistance. Also you have to realize that each generation of organisms are genetic variations from the parents through mutations which may give them an advantage over previous generations such as drug resistance. Resistance is a very real issue with anything... virus, bacteria and parasites. The more often exposed to meds (or herbs) the more likely the development of resistance, particularly if the meds are given in improper doses or durations, especially with parasites. Many parasites throughout the world have developed resistance to alot of the more commonly used modern meds due to over use. Couple well known examples are penicillin resistance of bacteria, MRSA (methcillin resistance staph aureus, drug resistant TB, and obviously HIV. Some experts predict that TB may again cause a world wide pandemic because some strains do not respond to any known meds. HIV has developed resistance to just about every med created to date... except kaletra (so far). Yes, resistance is REAL.