MODERN man owes it all to his ancestors. As he sniffs and sneezes, sweats and coughs through his latest bout of winter sickness, the fact he is still in this world at all is down to his relatives from the caveman days.

Every member of the family tree who ever caught a disease, and survived to rear children, passed on to those offspring a tiny defence mechanism to that particular ailment. So the sicklier your predecessors, the better your chance of surviving in a world still riddled with pestilence.

Experts believe that 98 per cent of the human genome - that's the genetic make-up of the body - is acquired from previous infections which have wracked the bodies of our forefathers and mothers.

The human body, then, is a superb bundle of agents designed to live with and fight a legion of diseases, whether they be viral or bacterial.

Unfortunately man, in his arrogance, has dabbled in the art of disease control and, by and large, has made things worse.

The sum total of scientific knowledge across all time has managed to get rid of only one disease from society. Smallpox was a killer. One in three of the many, many people who caught it died a horrible death, with skin, flesh and internal organs disappearing under a swathe of pustulent cankers.

An observant scientist in the 1790s, Edward Jenner, realised there was hope when he discovered that milkmaids, who contracted the less nasty cowpox as they performed their work, were immune to the killer smallpox variant.

A vaccine was born (in fact the word vaccine comes from the Latin word for cow - vacca), but it still took until 1976 for the disease to be officially eradicated - and only then because nature sided with man by not allowing it to be harboured in the animal kingdom.

It now only exists in vials, hidden deep in biological warfare laboratories in Russia and America, a hideous threat of the most barbaric kind.

Man did manage to put a bit of a dent in tuberculosis (TB) thanks to an immunisation programme begun after the Second World War. A hundred years ago, the respiratory disease was endemic in this country, claiming 50,000 lives a year. It was said that there wasn't a family in the land that hadn't lost someone to TB. Ignorance allows it to continue today as Third World nations fail to complete the obligatory six-month treatment programme, allowing TB not only to take hold again, but also to become resistant to drugs.

A half-hearted attack on malaria, one of the world's biggest killers, also exacerbated the situation. A massive programme took the fight to the mosquito larvae themselves, as their habitat was sprayed with oil and pesticide. Initially the scourge was reduced markedly but politics and war caused the scheme to fall apart, allowing the disease-carrying fly to fight back with a vengeance. As the bugs became immune to chemical sprays, the virus shrugged off the drug regimes.

Leprosy and the plague died off in this country largely through fear and circumstance, rather than scientific skill or knowledge.

Lepers were isolated and, as they died, so did the disease. Changes in lifestyle put paid to the plague. People decided that the squalid conditions they lived in, while good for the rats, were not terribly healthy for them. Get rid of the rubbish, the sewage, the thatched roofs and wattle walls and, suddenly, the rat and the disease-carrying flea it plays host to have nowhere to live. Drain the swamps, exterminate the rodents and disease goes with them.

But as one killer pathogen dies off, it is replaced by another, generally more virulent still and our bodies are fast becoming ill-equipped to deal with them.

"My granny always used to say 'a speck of dirt does you good'," says Dr Nicol Black, consultant in communicable disease control for Newcastle and North Tyneside Health Authority.

"You can keep yourself too sterile, which lowers natural immunity. Drugs being used erratically allows (germ) mutations to occur. If you don't kill the organism then they become resistant.

"There is also an overuse in humans and agriculture of antibiotics and, in future, we will not be able to do any invasive operations because we will not have the antibiotics to fight infections.

'Supermakets are now selling anti-bacterial sprays which is stupid. Washing is fine, so is soap and water, but not anti-bacterial agents which produce more resistant organisms."

Germs can also swap resistance. All manner of micro-organisms exist in the human throat, for instance, most of the time quite harmlessly. It's only when they combine with another organism - again something which might have existed benignly in another person - that a virulent reaction is triggered.

The e-coli outbreak which killed 21 people in Lanarkshire, Scotland, in 1996 only became lethal when it combined with a dysentery gene.

"The body has tremendous defences to keep things at bay which most organisms cannot breach. It's like a Mexican stand-off until you get a viral infection which allows them to penetrate. The mechanisms are like Star Wars, hugely complex and surrounding a competition between host and parasite," says Dr Black.

While he says the success of the meningitis C vaccine is good news, he warns that it doesn't mean the end of the disease.

"Any talk about eradicating meningitis C is premature. We have seen a dramatic reduction in cases where there has been immunisation, but it hasn't reduced the total number of cases. We have seen a worrying rise in the over 25s who haven't been immunised. So this is scant comfort to them."

There's also meningitis B, X, and Y to worry about while, in recent memory, meningitis A took lives by the thousand in the Himalayas.

Worse still are the innocuous flu viruses which regularly spread from migrating birds to pigs and on to humans.

The pandemic Spanish Flu, which swept the globe in weeks in 1918, killed between 20 million and 40 million people, many more than the war which paralysed the world at the same time.

In the 1940s, a flu epidemic spread through the British Army as soldiers were packed into barracks around the country. The disease was only halted after the men were ordered to move their beds 20 inches apart - too far for the germs to travel. Doctors realised the flu was being spread by snoring, which made the sputum at the back of infected throats airborne, passing it on to the man in the next bed.

With communications so good today, the spread of disease has never been easier. Two million people cross our national borders every day, many to and from exotic climes. And, according to Dr Black, the world is overdue its next pandemic, which will probably come from China and spread around the globe in weeks.

For the lucky ones, it will be a bug their ancestors have already had, while those that catch it and survive will do so to the benefit of their progeny. "We get exposed to germs a lot and we moan about it but it all goes into the immune memory," says Dr Black. "But viruses can evolve much faster than we can and will win in the end."