Viruses and bacteria are the ultimate parasites - and our only true predators.
John Tull fell into a coma
They have killed, maimed and destroyed the lives of more people than all wars put together.
More than any other single factor our history has been shaped by disease.
The Black Death has ravaged mankind for centuries, it has devastated every corner of the planet - at its peak in the middle ages it killed 25 million people in just five years.
For most people the plague is a thing of the past, but just last year John Tull almost became another victim.
Bitten by a rat in his back yard, John's body soon became the perfect breeding ground for the bacterium that causes the plague.
And despite giving him the most powerful antibiotics available, John slipped into a coma.
Plague causes tissue to rot
In a desperate bid to save his life his partner, Linda, gave the go ahead to amputate both of John's feet.
After 60 days John regained consciousness. He now has to learn how to walk again.
John's story is a reminder of how we can never underestimate the power of micro-parasites to strike when we are least suspecting them.
The final programme in the "Bodysnatchers" series reveals how, rather than reducing the threat from these micro-parasites, our technological breakthroughs are in fact making them stronger and providing them with the means to spread faster than ever before.
So how are we going to cope if a new superbug appears?
The bacterium that causes tuberculosis is the most successful parasite of all.
Billions carry tuberculosis
It is currently living in the bodies of 2 billion people - one in three of world's population.
There are records of TB victims dating back to the Egyptians (it was thought that Tutankhamen died from TB).
But with the discovery of antibiotics, TB's days should have been numbered.
For a few years the number of TB cases plummeted and then something very worrying began to happen - strains of TB resistant to the drugs began to appear.
Our own drugs had helped create these superbugs. TB began to reappear in cities where only a few years before it was all but eradicated.
In cities like London and New York, where people are crammed in and living closer than ever before, the TB parasite has found its perfect breeding ground.
Passed by coughing and sneezing, rates of TB are starting to approach epidemic proportions.
To try to contain the parasite we have resorted to the age old practices of isolation and quarantine.
The ability to change and adapt is what makes these micro-parasites such a threat.
Ebola kills up to 90% of its victims
And the parasitic masters of this are the viruses.
Every year millions of us are struck down by colds and flu, but still there is no cure. Able to disguise themselves by changing their "coat", viruses continue to baffle our medicines.
And they are extraordinarily adept at exploiting our technology against us.
The greatest threat facing us are the new viruses that are starting to emerge from remote regions of the world.
Lurking in jungles and swamps, these viruses have been contained to isolated outbreaks.
But with the advent of international jet travel, these parasites can escape from their remote lairs to every corner of the world in just a few hours.
The deadliest of these "emerging" micro-parasites are the haemorrhagic viruses.
Viruses that kill by causing their victims to bleed to death.
And the deadliest of these is the Ebola virus. First discovered just over 30 years ago, Ebola can strike without warning, and when it does it can rip through a community like a hurricane.
Transmitted by bodily fluids, the virus can kill within just a few days.
People stay away from Ebola funerals
At its worst Ebola kills nine out 10 people that catch it - the victims' bodies literally come apart at the seams. There is no treatment, although scientists are working to try to produce an effective vaccine.
The greatest threat humanity is facing is that one day a virus will emerge that can spread as efficiently as tuberculosis and that is as deadly as Ebola.
In February this year, it looked like our worst nightmare might just be about to become reality.
Severe acute respiratory syndrome, Sars, was a never before seen virus that was spread by sneezing and was killing one in 10 of the people that caught it.
Sars had the potential to kill millions, to defeat it would take something extraordinary.
But it wasn't a new wonder drug that saved us, but lessons that we'd learned from the past - hygiene, quarantine and, most importantly, sharing information.
Faced with the havoc that Sars could wreak, governments, scientists and doctors from around the world talked to each other, sharing vital bits of information and coming up with ways to beat Sars.
The Sars outbreak was a reminder of just how real the threat from these micro-parasites is, but it also showed that, if humanity stands together, we do have the power to defeat our most feared enemy.
Bodysnatchers was transmitted at 2100 GMT on BBC One on Wednesday, 10 December, 2003.