BEHIND A door marked "Biohazard" they are playing The Beatles. Seated at a laboratory bench, a white-coated researcher peers through a microscope as A Day In The Life reaches its noisy climax.

In a corner office, just to the right of a small inflatable dinosaur, Dr David Bhella is rhapsodising about viruses. "They are like snowflakes," he says, "geometrically and aesthetically fascinating. Rotavirus, for example, is a very common virus which causes diarrhoea in young children, and through a microscope it sort of looks like a wheel. That's a beautiful structure. Herpes virus is very beautiful, actually."

This is one of around 20 labs in the Medical Research Council's Virology Unit. Housed on Glasgow's Church Street in an ugly building with so many tiles missing from its facade it appears to be suffering from some disfiguring pox, the unit is Britain's leading centre for the study of viruses. In a room full of freezers - which since 9/11 has had a secure-card entry system - a quarter of a million live samples slumber at -70˚C. The scientists who work here are world experts in these microscopic miseries.

They know, for instance, that mankind is engaged in a constant struggle with viruses, our immune systems grappling with their protean proteins, and that we do not always win. Several hundred viruses affect humans; some are devastating. Measles claimed 345,000 lives in 2005. HIV infects a new person every six seconds. Deadly African viruses such as Ebola and Lassa fever, which can cause bleeding from the orifices, are on the increase as man pushes his way into new territories, and global business and tourism mean these killer bugs can make their way to the West. Viruses were once famously defined as "a piece of bad news wrapped in protein" and in 2007 bad news travels fast.

The men and women of the unit are not concerned primarily with prevention or cure; their role is to understand viruses at a more fundamental level - what they are and how they work. They specialise in hepatitis C, respiratory syncytial virus, and herpes. They are looking for an Achilles heel, the weakness in a virus which might allow it to be defeated, but are not themselves forging the weapons.

Twenty-seven scientists work here, plus postgraduates and support staff, and there is an almost tangible air of dedication and expertise. You might say these people live and breathe viruses. They are passionate about the subject, and their enthusiasm is infectious. Most admit to respecting viruses, the sort of feeling an athlete might have for a rival - appreciating their ability is an essential step towards defeating them.

Dr Samantha Willey joined the unit five months ago. She previously researched HIV at Edinburgh University, and although that virus is not studied by her new employers, she retains a sense of awe about its capabilities. "Once you learn more about the virus you just think how amazing it is," she says. "In comparison to humans, it is tiny - it has nine genes with which it can infect our cells, reproduce many more copies of itself and evolve to evade being found by the immune system, therefore enabling it to be passed to others. I know it seems strange to admire something so devastating, but HIV is perfectly adapted to infect us by exploiting the very immune cells which we use to get rid of pathogens and microbes. It's very clever."

Most of the scientists interviewed ascribe viruses human or animal characteristics, for example describing them as clever or wretched. "I respect viruses in the same way you might have respect for a lion," says Bhella. "It's beautiful but it can kill you."

The beauty of viruses is something he knows a lot about. An expert in electron microscopic analysis of their structure, he is looking forward to the arrival of a new microscope worth £780,000, which will allow the unit to study viruses in more detail than ever before. "Viruses give an insight into how small things can be," he says. "When you look at them under an electron microscope you use a magnification of typically 30,000-50,000 times. It seems abstract until you consider how many viruses could fit into a small space. For example, in a teaspoonful of sea water there are more viruses than there are people on Earth."

Two similarly jaw-dropping facts: an average virion (a single particle of virus) placed beside a flea is comparable to an adult human standing at the foot of a mountain twice the size of Everest; eight billion viruses could fit on a pinhead. Obviously these things are too small to be seen with the naked eye, and that's partly why seeing electron microscope images of their structure is such a thrill.

An exhibition of such images opens at Glasgow Science Centre this week. Molecular Machines is a collaboration between the virology unit and the artist Murray Robertson. On his office computer, Bhella scrolls through the astonishing exhibition. Cytomegalovirus is a pink and violet Jackson Pollock; hepatitis B resembles a tangle of malevolent fusilli; a cell infected with herpes simplex virus looks like the firebombing of Dresden as seen from above. It prompts one question: what exactly is this stuff?

A virus is a bundle of genes protected by a protein coat known as a capsid. Viruses cannot reproduce on their own and require the cells of another organism in order to do so. They are parasites, predators, pirates. Once inside the body, virions attach to cell receptors, penetrate and begin replicating. They turn each cell into a factory for manufacturing viruses, which then leave that cell, sometimes bursting it open, and go on to infect others.

It can be frighteningly efficient. If one particle of rhinovirus - the common cold - gets inside a cell in your throat, it can replicate itself 10,000 times in a single day. Then those 10,000 virions infect a further 10,000 cells, and so on. Rhinoviruses have only 11 genes, human DNA has around 25,000 and yet we are at their mercy. Adults usually catch two to four colds every year, we sneeze the virus out, and it goes on its way to the next person. Indeed the most devious thing about the virus is that it makes us develop symptoms that aid its spread, but not so sick that we have to go to bed, away from other potential hosts.

"Speed and numbers are the two key factors for the success of viruses," explains Dr Frazer Rixon. Some reproduce so quickly and in such quantities that they use the evolutionary process of natural selection to their advantage, mutating faster than the human immune system can keep up.

This isn't such a problem with the common cold, but when it comes to more serious viruses this mutability can be devastating. Influenza is estimated to infect one billion people annually, leading to between 300 and 500,000 deaths. Major flu pandemics, such as the 1918 outbreak which claimed the lives of more people than had died in the first world war, occur when the virus makes a sudden genetic change, becoming unrecognisable to antibodies that a person has acquired from previous infections.

Although these pandemics are mercifully infrequent, experts believe we may be on the brink of another, possibly caused by H5N1, the bird flu. The World Health Organisation estimates that if H5N1 emerges as a fully contagious virus (at present it does not jump easily between birds and humans, or spread readily among us) then it could reach all continents in less than three months and kill anything from 2 to 7.4 million people, a conservative estimate.

"Sooner or later there will be a new influenza virus and it will be a large killer," confirms Virology Unit director, Professor Duncan McGeoch. "It has happened regularly every few decades for as far back as we can characterise flu, and there's no reason to think that has stopped."

McGeoch led the team that sequenced the genome of herpes simplex virus type one (in other words identified its genes). Understanding the genetic structure of viruses is a key strategy in the war against infection, and work done at the unit is reaching a point where it may be possible to prevent reactivation of latent herpes simplex viruses, which cause painful blisters on the lips and genitals.

Latency is a stumbling block for virologists. If we think of viruses as criminals, then those which use mutation as their primary strategy are masters of disguise, evading antibodies by concealing their identities. Viruses which use latency, however, are more like burglars who hide inside a bank they plan to raid, emerging only when security - the human immune system - is weakened, and wreaking violence in the vaults.

"How do we get at these latent viruses?" Dr Chris Preston asks, rhetorically. "It's hard for people with genital herpes. You can treat the blister, but the virus itself is in the person's nerve cells, just sitting there, and you can't get at it." Will it be accomplished one day? "Yes," he says. "It's a difficult problem. You've got to try and distinguish the virus from the cell it's in. But our basic philosophy is that if you keep working and understanding then it will be done."

"Science is not about great people making great discoveries every day," says McGeoch. "It's about coming to work and slogging on. Experimentation has to be worked at very hard. You have to keep knocking your head against the wall until it falls down."

In the virology unit, the sound of cranium against plaster emanates loudly from those labs concerned with hepatitis C virus (HCV), an infection which can cause chronic liver diseases including cirrhosis and cancer. "In terms of numbers of people infected, it's a much bigger problem than HIV," says John McLauchlan, who leads a team dedicated to HCV.

Hepatatis C virus has infected 170 million people worldwide, but is also a particularly local problem. Half a per cent of the UK population are thought to be infected. In Scotland the rate is 1%, and in Glasgow 2.5%. HCV is spread primarily through direct contact with infected human blood, and in Glasgow that happens for the most part when intravenous drug users share needles. There is a six out of 10 chance that the person whose needles they use is already infected.

There is no vaccine against HCV and current anti- viral drug treatments are problematic. They only work in 40% of cases, are costly, and can cause depression. "So the scale of the problem is increasing," says McLauchlan. "We don't know how many infected people will progress to serious liver disease. If they do, that puts a significant burden on healthcare services. For those who don't respond to treatment and develop serious liver complications, a liver transplant may be the only option."

Even with a donated organ the patient is not cured. The virus remains in the blood and the new liver becomes infected almost immediately. Worse, while it may have taken 20 years to progress from initial infection to chronic disease, this process will be significantly shorter second time around. "At some point we hope to have a magic bullet that will stop the effects of the virus, and there is a huge amount of work going into that," says McLauchlan. "But the current UK estimate is that this will be a pressing problem for at least 20 years."

The search for a cure is complicated by the fact that hepatitis C is tricky to study. For one thing it's quite dangerous; in the unit the work goes on in a special restricted-access containment facility kept under negative pressure, meaning that when the door is opened all the air flows into the lab rather than out. Also, it has only recently been possible for scientists to grow HCV and it remains difficult; if you can't actually follow how the virus spreads from cell to cell then it's hard to know how to target it.

It must be easy to become disheartened, but speaking to people infected with the virus keeps McLauchlan motivated. "If you are infected with hepatitis C the standard treatment is a combination of Interferon and Ribavirin. We can study our cell lines here and treat them with those drugs and see the effects on the virus; that all seems good and well, but when you talk to people who take the treatment you understand it's not pleasant and many come off the therapy as a result. Meeting those people gives you a human perspective on the disease, and you hope that some of the work you do leads to treating people."

Spend time thinking about viruses and larger questions emerge. What are they for? Where have they come from? Their evolutionary origins are open to speculation. The accepted thinking is that millions of years ago viruses began as genetic material which somehow broke away from the genome of a primitive form of life. Viruses then evolved alongside their host organisms, and may actually have played a role in the evolutionary development of the hosts.

Humans, for example, have viruses within our DNA. Some 8% of our genome is made up of ancient human endogenous retroviruses (HERVs) which at some point in the distant past used molecular tricks to insert themselves into our genetic structure and have then been passed down. In this sense, we are actually part virus. This is not to say that the HERVs act directly in specifying composition of our bodies, but arguably we do need them inside us.

So, finally, what are viruses for? The "intelligent design" movement argues that nature is too complicated and intricately balanced to have arisen accidentally, and therefore must be the creation of God. If we assume this mindset for a moment and consider viruses - structurally perfect entities which can cause pain, misery and death - then it's tempting if fanciful to see some sort of Satanic intelligence at work.

"You can't think of a virus as good or evil," insists Rixon. "It is not there for our benefit, but for itself. The imperatives of a virus are to reproduce, metabolise and survive. They don't go around eating grass, and I'm sure they don't have romantic attachments, but at a basic level they are doing what all living things have to do, and are perfectly designed to do so or they wouldn't be here. They've been around for the whole history of life on Earth, and in terms of numbers and diversity this is the viruses' planet not ours."

That is not to say viruses do not have some functions which benefit mankind. For instance, bacteriaphages are viruses which target bacteria. They are probably the most populous organism on the planet, and without them we would be completely overrun by bacteria. We have not had much success in wiping out viruses, smallpox being the only one to be completely eradicated, but were we somehow able to make our planet virus-free then there might be negative consequences. One unit scientist speculates that without viruses to occupy them, our immune systems could start attacking our own bodies.

Thankfully, that problem need not concern us for the moment. As the continuing HIV disaster demonstrates, even decades of focused attention by the best minds on the planet cannot outwit viruses. So, while the quest for anti-virals and vaccines continues, perhaps our best hope is to harness viruses to our own advantage, using them just as they use us. A recent news story suggested that in future we might actually be able to use viruses to fight cancer. British scientists have been exploring the possibility of killing tumours by infecting them with adenoviruses, a virus associated with the common cold.

"The caveat," says McGeoch, "is that viruses have been evolving for millions of years and all the subtleties included in a virus genome are mostly still unseen by us. Just changing it from a baddie to a goodie is fairly challenging. But people are working hard at getting a virus that could destroy a cancer cell and it's not to be sneered at."

He laughs, however, when asked whether he can see some point in the future when we will understand viruses totally. "Most of the virology of today has only developed in the last 50 years, and what I would classify as total understanding of even the simplest virus is a long way off."

So it seems that for the moment viruses must remain mysterious enemies within. They are simple, yet extraordinarily sophisticated and the clearer a view we get of them the more brutally effective they appear. Even the humble common cold, when looked at in detail, seems astounding. To paraphrase The Rolling Stones, it makes you blow your nose and then it blows your mind.

Molecular Machines - Images From Virus Research is at Glasgow Science Centre, February 3-April 30, before touring in Edinburgh, Dundee and Aberdeen