Why should I buy an anti-parasite drug?
Many products can be very effective against parasites. But do they work against all parasite? If there is no control group of the population against which to measure effectiveness, then no matter what the drug is, its results cannot be taken as a valid predictor of what the population's response to it will be.
The answer is that the effects of drugs can be very small. If the effect is less than 10%, then they will only be seen in a few cases. But it is difficult to measure the effect in any meaningful way. The effect may be much greater than 10%, but it will be impossible to measure. The question is whether the small effect of a drug will make it worthwhile to try another one. It is a well known principle of statistics that an experiment is worthless if the results from the experiment are not statistically significant. The use of the term parasite is particularly misleading. The term was coined by the late Edward C. Merril. The term was originally intended to mean "living parasite," but it was never used in that sense.
Why do some parasites live more than a decade before they die and others live 10 years or longer? They can live for years and then die. When this happens, the parasite can have an offspring and keep growing.