The bug was present in victims' brains

A control study of 19 people who had died from other causes found the bug in only one person's brain. It was much less likely to spread in this person.

In the Alzheimer's patients, the bug was found in the two areas of the brain most likely to be affected by Alzheimer's: the hippocampus and the termporal cortex.

In addition, the researchers managed to culture the bacterium from two of the Alzheimer's affected brains and found that it was still active and alive.

Risk factor

The research, published in New Scientist magazine, suggests that C. pneumoniae, which may also cause narrowing of the arteries and heart attacks, may be a risk factor for Alzheimer's.

The researchers say the bacterium alone could not cause the disease.

C. pneumoniae causes inflammation in other parts of the body, including the lungs, and the brains of Alzheimer's patients are known to become inflamed.

The scientists believe the bacterium attacks the immune system, causing it to produce chemicals called cytokines which trigger inflammation.

The researcher do not know why the brains of people who develop Alzheimer's are more vulnerable to the bacterium than others who carry it.

Around 400,000 people in the UK have Alzheimer's, including one in 20 people over the age of 65.

It causes progressive memory loss and confusion.

Caution

The Alzheimer's Disease Society was cautious about the new research, saying it was a small scale study.

A spokesman stressed that the disease was not contagious.

Other British experts also urged caution, said Alzheimer's patients teneded to have a lot of diseases present in their bodies when they died and called for further research.

Some commentators have suggested the findings could mean that antibiotics could eventually be used to treat Alzheimer's.

However, experts say it is far too early to say if this is the case and that C. pneumoniae is not thought to respond well to antibiotics.