Why antibiotics for bacteria but not viruses?

Chris - It's a very good question.

The answer is that bacteria are single-celled organisms. They are living, alive and have a metabolism.

Viruses are the ultimate parasite. They're not really alive. They are an infectious bag of genes which are absolutely tiny. A flu virus is one ten-thousandth of a millimetre across, and they're so tiny that they do not have any of the machinery inside them to make new copies of themselves. They have to infect a cell in order to do that.

That means you've got a problem, because bacteria look totally different to our own cells, so it's fairly easy to make drugs and chemicals that will roger a bacterial metabolism and which will not affect our own metabolism.

Because viruses have to prey on our own metabolism, and they have to use our own cells to make copies of themselves, it's very difficult to find ways to discriminate between the virus and a healthy cell and therefore avoid side-effects.

There are some drugs that can do that. The most famous is a drug called acyclovir; most people will know it as "Zovirax", which you put on cold sores. This works because the drug is a special chemical which is activated only in the virally-infected cell.

The virus makes an enzyme, which locks onto the drug molecule and it switches it on. It will not get switched on in any other cell, and once the drug is switched on it forms a special DNA "letter" which when the virus incorporates it into its DNA it cannot make the DNA chain of the virus grow any more. The chain terminates the virus and stops the virus growing its own DNA.

It's very difficult to do this. This thing that researchers are now looking at is the possibility of something like RNA interference. This is where you make short pieces of genetic material which are the mirror image of the virus's own genes. By putting those into the cell they lock onto the viral genes and make what's called double stranded RNA.

Cells usually associate this with rubbish, junk or viral infection, and they target them to the cellular equivalent of the wastepaper basket and it gets ditched. That's how you can switch off viruses in cells. Our own immune system uses that strategy as well sometimes.

Overall, it's a key problem that we've been grappling with for a long time. That's why we haven't got a cure for the common cold, I'm afraid!