TL;DW:
Due to various molecular configurations (aka polymorphs), currently known medicines when contaminated with a catalyst seed molecule/particle can become irreversibly unreproducible via chained-contaminations.
- the contamination can cause the existing configurations to become more stable in a negative fashion where the medicine/compounds no longer work they way we expect them to.
This system of contamination and reproduction acts in similar fashion to viral infections.
Current Solutions:
- better clean room procedures for all steps of medicine manufacturing.
- pour more money into medicine polymorph research.
For protein based drugs there are prions: misfolded protein isoforms able to catalytically convert correctly folded proteins, basically following the same logic as the examples of smaller molecules. So far I guess we only know of one group of proteins that do this (like the mad cow disease one) but theoretically it could always happen to others we just don't know about yet. :)
Sorry if that made anyone feel worse again..