Bacteria and viruses constantly evolve to develop drug resistance. Is there a limit as to the number of drug resistances they can maintain? The DNA of bacteria and viruses may be too simple to remember all of the drug signatures. In other words, their DNA may hold only a limited number of the drugs that they have encountered in the past.
If so, the drug that a bacterium or a virus encountered some generations ago in its mutation cycle may become effective again, while the recent drugs are still ineffective because of the recent memory. The situation is somewhat analogous to the human immune system becoming ineffective against smallpox vaccine after a few decades.
