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Viruses are a highly diverse group of unique noncellular pathogens that invade cells of all types of organisms. Although they are nonliving, viruses replicate within living host cells through a multistage life cycle. Viruses are primarily classified by analyses of the nucleic acids they contain and how the viral genomes are replicated. Some viral infections are short term, while other viral infections can be persistent and cause cancer or permanent genetic damage. Bacteriophages, viruses that infect bacteria, typically follow a lysogenic life cycle that differs from the lytic life cycle common in animal viruses. Viroids and prions are noncellular infectious agents that resemble viruses in certain ways but are very distinct from viruses.

At A Glance

  • Viruses are unique and nonliving infectious particles that invade cells. The viral genome is surrounded and protected by a protein coat called a capsid.
  • Because of the multitude of unique properties that different viruses contain, classification of viruses is complex. With no kingdom, viruses are classified according to seven orders and over 100 families.
  • Viruses change the metabolism of their host in order to replicate, often killing the host in the process. This occurs through a life cycle involving multiple steps, each with specific processes that ensure viral survival.
  • Viruses can have effects that span over days, weeks, months, or years, including persistent infections—those that can last a long time—and transforming infections—those caused by oncogenic viruses that permanently damage genetic material.
  • Bacteriophages infect only bacteria, and some have a lysogenic life cycle, whereas many animal viruses go through the lytic life cycle.
  • The main reasons for cultivating viruses include isolation and identification, preparing viruses for vaccines, and carrying out detailed research.
  • Viroids, like viruses, are noncellular infectious agents, but they infect only plants and are made of a single-stranded RNA molecule.
  • Prions are another example of unique noncellular infectious particles composed entirely of protein and are known to infect animals and cause hereditary, sporadic, or infectious neurodegenerative diseases in humans.