Many antiviral medications are effective at killing only some strains of the targeted virus. If only some strains of a targeted virus are killed by a given medication, there is a danger that, through random mutation or the emergence of other strains from natural reservoirs, resistant strains will gradually dominate those vulnerable to medication. If the antiviral medications are continually administered, for example to humans or cattle, there is a significant risk that existing medications will grow ineffective, leaving doctors with no remedies for these diseases.
The conclusion of the argument above depends on which of the following assumptions?