What is a Virus? What are Viruses made up of ?

4. How do Viruses Cause Disease?

Most viral infections do not result in disease, but a small number can cause extensive damage. This can be either because of the virus action against the cell, or the immune response to infection, and both can lead to symptoms. Indeed, symptoms can develop a number of ways from mild, self-limiting disease, to severe or lethal in the absence of treatment. Viral diseases in humans range from the common cold and warts to more profound, life-threatening syndromes such as measles, mumps, influenza, HIV, a range of enteric illnesses and haemorrhagic fevers (such as Lassa Fever, Marburg or Ebola). The severity of outcome will depend on the infectivity of the virus, but also on several host factors, including variation in cellular receptors and immune status.

The immune response is of course essential, but it can also be a problem. When viruses infect cells, they shut down the normal cellular processes, disrupting regulatory balances to focus on replication and synthesis of new virus particles. This causes a major stress response in the infected cells that can trigger cell death (apoptosis, cell “suicide”), but also sends out signals (cytokines) to alert the immune response. Cytokines attract immune cells to the affected area to kill virus-infected cells and trigger the production of antibodies that can neutralise the virus. However, sometimes this response can go into overdrive, causing a cytokine storm, which leads to damage of healthy as well as infected cells.