On page 2, we saw that there are four main types of microbial infection. We looked at bacteria and fungi. On this page, we will look at viruses and on page 4 we will examine protozoa.
Picture 7. There are different types of viruses that cause a range of diseases in plants and animals. Viruses are a tiny ball of protein thousands of times smaller than a cell.
Virus
Viruses are the smallest known organisms. Viruses cause diseases including the common cold, influenza (flu), measles, mumps and rubella.
Measuring just a few thousand millionths of a metre (nanometre), they are little more than a ball of protein containing a small section of DNA. They are unable to reproduce without the assistance of the host cell that they infect. Viruses take over a host cell which is then directed to make copies of the virus until eventually the cell bursts. In this way, viruses kill infected cells, spread and damage body tissues.
Picture 8. Animation showing
how a virus infects a cell. Open
the animation in a new window.
Viral infections
Viral infections are difficult to treat because the virus reproduces inside living cells. To destroy the virus, the cell must also be destroyed. Antiviral medicines, such as those used in the treatment of HIV/AIDS, are often targetted at stopping the virus infecting new cells and preventing the virus from reproducing.
The body's immune system recognises any viruses it encounters and can launch an immune response to fight off the infection. Unfortunately this takes several days to become effective and the infection can cause harmful symptoms before the immune response is large enough to destroy all of the viruses in the body. With many viral infections, the patient may simply told to rest and take aspirin to reduce any fever until the body recovers from the infection.
Prevention is better than a cure
One method to prevent infection from viruses is to vaccinate against the virus. You can find out more about how vaccination works in The human immune system. Vaccination prepares the body's immune system for any future infection by the virus. The body will then be primed to destroy the virus before any infection can occur. Work is currently under way to produce a vaccine against the virus that causes HIV/AIDS. This is a difficult task as the Human Immunodeficiency Virus (HIV) attacks the cells of the immune system itself.
Some viruses are now being turned to a useful role. Experimental treatments for genetic disorders, such as cystic fibrosis, are using modified viruses to carry healthy genes to replace the faulty genes in the affected cells. This stimulates the cells to use the new gene to make the protein that would normally be missing.
On page 8, you can find out more about SARS, a viral disease.
New viruses are made in the host cell's cytoplasm.
