Trillions of alien cells found hiding in human body

Scientists have compiled a list of every microbe found in a healthy human body. The conclusion: there are trillions of them. And we need our parasites as much as they need us.

What would a totally germ-free human be like? Radiantly healthy? In fact, the answer is just the opposite: without our germs, we would not last a day.

In fact, of the trillions of cells in a human body, fewer than one in ten are actually human. The others are from parasites: bacteria, fungi, even worms. It has long been suspected that these organisms play a vital role in our system. Now, a newly compiled scientific map of this ‘human microbiome’ has proved this once and for all.

These alien organisms are tiny compared to human cells, and make up only a small fraction of our total mass, but according to US scientists from the government-funded Human Microbiome Project, they contain over half of the genetic ‘tools’ humans need to survive. Bacteria in our tongue and gut break down food. ‘Good’ viruses help our immune system to fight off ‘bad’ ones. The right kind of bacteria can even protect us from stress and depression.

For millions of years, people and their parasites have evolved side by side. ‘If someone didn’t have their microbes,’ one professor says, ‘they’d be naked.’ But since the mid-19th Century, when humans discovered that germs were responsible for disease, we have have been waging a deadly war on microorganisms.

This war has produced cures for some of humanity’s worst plagues: smallpox, cholera, polio. But biologists are beginning to realise that some healthy microbes may be in danger of being caught in the crossfire.

The microbiome mapping project has revealed that some potentially deadly microbes are actually very common. Over 30 percent of the subjects they studied, for instance, hosted the SARS virus. It was certainly doing them no harm – and may have been fulfilling useful functions.

What is more, our immune system builds strength by fighting minor maladies like the common cold. When our food, water and environment is too pure, our immune system remains weak – probably one reason why allergies are so much more common today than in the past.

Even the skin-burrowing hookworm may be on our side: it has been effectively used as a cure for asthma.

Watch your bacteria

Fascinating, say some scientists. But there is no need to get sentimental: some bacteria might help us, but on the whole our war on disease has yielded spectacular results. In 1850 the British life expectancy was 43, they point out; since we started targeting germs, it has risen to 80.

But the researchers say that our quest to ‘cure’ every illness in the world could end up disrupting the balance of our microbiome. Ease off on the hygiene, they say: we are are messing with a delicate and finely-tuned system.

You Decide

  1. Are you disgusted by the idea of sharing your body with microbes?
  2. Have we become too obsessed with hygiene?


  1. Imagine you are the first person to look under a microscope and discover that the world is full of invisible organisms. Write a diary entry for the day of your discovery.
  2. Draw a diagram of a microbe, with labels for the each of the following features: nucleus, cell membrane, vacuole, cytoplasm and food storage granule.

Some People Say...

“Cleanliness is next to godliness.”

What do you think?

Q & A

So I don’t need to worry about washing my hands any more?
Yes, you absolutely do! There are still plenty of bacteria in our environment that are simply bad for us, and the essential ones are already in our body. It ispossible to be too hygienic, but to do this you have to be truly obsessive about cleanliness – to the extent that it is a mental illness.
In that case, will these discoveries actually change anything?
They could have an enormous effect. For instance, injecting the intestinal microbes of a healthy human into someone with gut problems can cure them; and scientists still have no idea how this works. In the future, we might be able to feed the good bacteria and destroy the bad ones, cultivating in our body the perfect system of microbes.

Word Watch

The idea of the ‘microbiome’ comes from the argument that the microbes an organism hosts should be included as an essential part of their genetic makeup (or genome). Though originally applied to humans, it is now used to describe microbial cultures in everything from bats to seawater to soil.
Immune system
One of the body’s major systems, our immune system is what protects us from disease. It is made up of specialised cells called white blood cells or ‘leukocytes,’ which are broadly split into two types: one type that engulfs invading cells, and another that remembers and recognises those that are unhealthy, so that methods can be found of destroying them.
Severe acute respiratory system, or SARS, is a serious flu-like disease that can kill those who are already weak from illness or old age. There was a serious scare in 2002-3 when SARS broke out in hospitals, causing hundreds of fatalities. For now, however, the disease is fully contained.
Common cold
Not actually a disease but a ‘syndrome’; that is, a bundle of symptoms with many different causes. Sometimes colds are a very mild dose of influenza, other times they are an excess of normally healthy bacteria. This is why the cold has proved so impossible to cure.


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