How do viruses cause disease?

Chris - Viruses are not actually living - they're just an infectious bag of genes; in other words, they are some kind of coat that surrounds infectious material. This can be DNA, which we've all heard of - we've all got that - or its genetic relative called RNA.

Viruses are also tiny: a flu virus is about 1/10,000'th of a millimetre across. That's too small for them to have any of the machinery they need to make new viruses inside themselves, so they need to hijack a cell to do that. There are viruses for plants; there are viruses for animals, and there are even viruses for bacteria [called bacteriophages] - so bacteria can catch a cold too!

The 'flu virus has "receptors", which are like viral Velcro, on the surface of the virus particle. They will lock onto a target cell using these chemical receptors on the surface, which docks onto the cell surface. They then go into the cell. Once they're in the cell they use it like a factory: they take it over and make it produce thousands - or in some cases millions - of new copies of the virus, which come streaming out of the cell. They infect other cells to make more viruses, or they escape from the body and infect a new victim. When they're damaging cells - when they're infecting cells - they can potentially kill them - that's called lytic infection. When they kill a cell that has a consequence for us because if it's a cell in your airway, for example, it might damage the mucosa (the lining of your airway). This means you get inflammation, a rather blocked-up, sniffy nose. Plus, because you've got damage to the lining of the nose, you might get a bacterial infection on top, so they can cause secondary infections.

Kat - Also, something like Ebola. Ebola's a virus where it just breaks down the tissues of the body...?

Chris - Yes, it depends on the "tropism". It depends on what sort of cell the virus targets, because if the virus goes into the cells in your respiratory tract then it can damage the respiratory tract. 'Flu can damage the lungs, and this can cause respiratory failure. Other viruses have a tropism towards other tissues. For example, HIV has a tropism towards cells which have a "CD4" chemical marker on them.

Kat - Those are immune cells?

Chris - Those are immune cells, and so the virus goes into those cells. It can loiter in the cells for a long time before it actually does infect them, but often it can damage the cells and make them die. If you lose those cells, your immune system is disabled.

So there's a whole host of ways in which the viruses can damage different parts of the body. When you've got polio virus, this comes out of your intestines, goes into your blood, and then goes to your spinal cord. It then invades motor neurones, which are the nerves which supply your muscles, and the virus grows in the motor neurones, killing them in the process. This paralyses you.

In summary, it depends what sort of cells the virus is targeted at to determine how likely it is to cause damage to that tissue and how likely it is to have consequences that are clinically manifest...