Like bacterial virulence factors, viral virulence factors help the virus spread, replicate or evade the immune system. Some major virulence factors are summarized in the table below.
| Virulence Factor | Effect |
|---|---|
| Viral oncogenes | Insert into the cell genome and activate proto-oncogenes in the target cell |
| Immunoglobulin proteases | Breaks down immunoglobulins to prevent a host response |
| Capsid and envelope proteins, hemagglutinins | Used in attaching to cells and act as antigens to stimulate an inflammatory response |
| Viral enzymes | Used in replicating viruses |
| Viroporin, phospholipids, endolysins, neuraminadase | Form pores in the cell membrane |
| Viral fusion proteins | Form syncytial cells (fusion of two adjacent cells) |
| Apoptotic proteins | Activate or inhibit caspases to control apoptosis |
Although not an virulence factor per se, inclusion bodies are a visual representation of viral proteins that can be seen under the microscope. RNA viruses tend to produce cytoplasmic inclusions, while DNA viruses produce intranuclear inclusions.
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