Antiviral drugs can work byInterfering with the replication of viruses and strengthening the immune response to the viral infection. Most of the drugs used to treat human immunodeficiency virus (HIV) infection. Because viruses are tiny and replicate inside cells using the cells' own metabolic functions, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are relatively large organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions that antibiotics can target. Therefore, antiviral drugs are much more difficult to develop than antibacterial drugs. Also, unlike antibiotics, which are usually effective against many different species of bacteria, most antiviral drugs are usually effective against only one or a very few viruses.