Why Do We Fall — Science STD 9 — Question

Viruses have few genes, sometimes less than 10, and thus few proteins that can be targeted. By contrast, bacteria have thousands of different genes and proteins. There are just more targets to shoot at with bacteria. Bacteria (except in spore stages) must be continuously active metabolically, and thus can be poisoned at any time. Viruses are metabolically inert until they have already infected a host cell, and are not susceptible to the actions of antivirals until then. An exception would be drugs that block viral adhesion to host cells, thus preventing their entry.
Viruses use a lot of host cell proteins for their replication and spread. Any drug that targets host cell proteins will be toxic to the host. Viruses often hijack host genes, and thus their proteins can be closely related to host proteins. Making a drug that discriminates between viral and host versions of a protein is very very difficult. Viruses are far more diverse than bacteria. Whereas most antibiotics are effective against many different species of bacteria, antivirals usually are very specific to one type of virus.