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Infectious Diseases

Infectious Disease information and resources for the MSK community, including clinicians, patients, and the general public.

What are Viruses?

A virus is a small infectious organism—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell (called the host cell), enters the cell, and releases its DNA or RNA inside the cell. The virus’s DNA or RNA is the genetic material containing the information needed to make copies of (replicate) the virus. The virus’s genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells. Viruses are classified as DNA viruses or RNA viruses, depending on whether they use DNA or RNA to replicate.

Types of Viruses

DNA Viruses

A DNA virus has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase.

RNA Viruses

A RNA virus has RNA as its genetic material and replicates using virally encoded RNA-dependent RNA polymerase (RdRp).


A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell.

Virus Classification

International Committee on Taxonomy of Viruses (ICTV) Classification of Viruses

The ICTV classifies viruses into seven orders based on morphology:

  • Herpesvirales - large eukaryotic double-stranded DNA viruses
  • Caudovirales - tailed double-stranded DNA viruses typically infecting bacteria
  • Ligamenvirales - linear double-stranded viruses infecting archaea
  • Mononegavirales - nonsegmented negative (or antisense) strand single-stranded RNA viruses of plants and animals
  • Nidovirales - positive (or sense) strand single-stranded RNA viruses of vertebrates
  • Picornavirales - small positive strand single-stranded RNA viruses infecting plants, insects, and animals
  • Tymovirales - monopartite positive single-stranded RNA viruses of plants

Baltimore Classification of Viruses

The Baltimore classification groups viruses into seven categories based on how they produce mRNA:

  • Group I: double-stranded DNA viruses
  • Group II: single-stranded DNA viruses
  • Group III: double-stranded RNA viruses
  • Group IV: positive sense single-stranded RNA viruses
  • Group V: negative sense single-stranded RNA viruses
  • Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle
  • Group VII: double-stranded DNA viruses with a RNA intermediate in their life cycle