"In the case of infectious disease, clearly your scenario isn't what happens in the real world. Otherwise no disease could be cured." Note that your statement

"In the case of infectious disease, clearly your scenario isn't what happens in the real world. Otherwise no disease could be cured." Note that your statement is only true for a relatively narrow range of infectious diseases, and mostly in special cases where we have "Nuke the site from orbit" class drugs that don't target things eukaryotic cells use.