Viruses are a class of tiny and deadly infectious agents that are distinct from other types of germs such as bacteria. Because viruses are not composed of cells like other organisms, they are not even considered to be alive by many scientists. Instead of being a single-celled or small multicellular organism like most other pathogens, they are composed of a strand of DNA or RNA encased in an envelope of protein, called the capsid. Some viruses also posses a protective layer of fat over the capsid. Because of their unique structure and method of causing disease, viruses cannot be treated with antibiotic drugs but must instead be treated with a class of drugs referred to generally as antivirals.
How a virus infects a host is crucial to understanding how infections begin after individual virus particles infiltrate the host's cells. This can be accomplished because viruses are so tiny that they may slip through cellular defenses unopposed. Once inside a host cell, a virus moves to the cell's nucleus where all the DNA and RNA functions as instructions for the cell's operation. By inserting its own genetic information into the cell's nucleus a virus can hijack its function, causing it to produce and release more virus particles so the infection spreads. Another common effect of this cellular hijacking is the damage, breakdown and eventual death of the cell itself.
Like most drugs used to treat infectious pathogens, antivirals are targeted to specific strains of viruses and work in a variety of ways. Most antiviral drugs don't actually kill the virus particles themselves as inhibit their reproduction. Since viruses cannot reproduce without infecting a host cell antiviral drugs have been designed to interfere with the infection process.
This interference may be achieved in numerous ways, including blocking the virus from the host cell, preventing the virus from releasing its genetic material once it reaches the nucleus and preventing the virus's genetic data from being spliced into the host cell's DNA. Various highly specific antiviral drugs have also been developed that target the enzymes and proteins that an infected host cell uses to assemble new virus particles and prevent them from functioning correctly. Such drugs must be designed very carefully so that they do not interfere with the metabolism of healthy cells. A final type of antiviral drug targets the virus indirectly, by increasing the efficiency with which the host's immune system can fight the viral infection.