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PH-IM08

How viruses injure tissue

How a virus takes over a cell, why some viruses kill cells directly while others injure tissue mainly through the immune response, how some hide for life, and how a few push cells towards cancer.

By the end, you can

  • Describe how a virus enters a cell and uses its machinery to copy itself, and what decides which tissues it infects.
  • Explain the direct cytopathic effects of viruses, including lysis, syncytia and inclusion bodies.
  • Explain how the antiviral immune response causes symptoms and tissue damage.
  • Distinguish acute, latent and chronic viral infection, with examples.
  • Explain how oncogenic viruses such as high-risk HPV push cells towards cancer.
  • Recognise the classic inclusions, syncytia and tests of viral infection, and explain how antivirals and vaccines act on steps of the viral life cycle.

What it is

A virus is a small infectious particle made of a DNA or RNA genome in a protein coat (capsid), sometimes wrapped in a lipid envelope. It has no metabolism of its own and multiplies only inside living cells. Viruses injure tissue directly, by killing or altering the cells they infect, and indirectly, through the immune response that clears them; some persist for life, and a few contribute to cancer.

4 types to tell apart
Acute infection
The virus multiplies, damages cells and is cleared within days to weeks, as in influenza or rotavirus.
Latent infection
The genome stays silent inside cells, making little or no virus, and can reactivate later, as with herpesviruses.
Chronic infection
The virus keeps multiplying for months or years despite the immune response, as in hepatitis B and C and HIV.
Transforming infection
Viral genes push infected cells to divide and survive, contributing to cancer, as with high-risk HPV.

Normal function

A cell’s machinery and its alarms

Every cell keeps its DNA inside the , transcribes genes into messenger RNA and translates it on ribosomes; proteins for the surface and for export are made on the and leave through the . A virus brings only a genome and a few proteins, so it must borrow all of this.

Cells also watch for intruders. Sensors in the cytoplasm and in endosomes recognise viral nucleic acids, such as double-stranded RNA, and switch on type I interferons, which warn neighbouring cells to slow protein synthesis and break down viral RNA. Natural killer cells kill cells that show signs of stress.

Every nucleated cell also displays fragments of its own proteins on MHC class I molecules. When a virus is making proteins inside, viral fragments appear there too, and cytotoxic (CD8) T cells that recognise them kill the cell. Antibodies neutralise virus outside cells.

Point to it on the model