Chlamydia is already known to cause considerable human misery. Not only is one strain of the micro-organism responsible for Britain’s “epidemic” of sexually transmitted disease, but another variant can cause a serious respiratory-tract infection similar to Sars.

Now comes the surprising finding by a German research team that chlamydia may be linked with schizophrenia. Dr Rudolf Wank, an immunologist at the Ludwig-Maximilians-University in Munich, has reported recently that schizophrenic patients are much more likely to be infected with one or more variants of chlamydia. More importantly, he found that targeting the bug with specially treated immune cells improved the patients’ symptoms dramatically.

About 40 per cent of the 75 patients he studied were infected with chlamydia, compared with 6 per cent in the control group (ie, people who did not have schizophrenia). As Dr Wank explains: “Chlamydia comes in three varieties, two of which can cause a flu-like respiratory infection or pneumonia, while the third causes the sexually transmitted disease. The patients were much more likely to have one or more of these.” The team also found that the risk of developing schizophrenia rose dramatically for patients with a certain group of immune system genes.

“Kurt” had been on antipsychotic medication for more than 20 years; he was aggressive, his speech incoherent and he was unable to work. In a report published in the journal Medical Hypotheses, Dr Wank described how, after identification of the strain of chlamydia infecting him and treatment with “treated” immune cells, within a few weeks Kurt’s aggression had disappeared and his speech had improved.

The idea that an infection might cause mental illness has a long history — it is well known, for instance, that certain conditions can have a psychological effect. Hepatitis B may cause depression and the microbes causing syphilis and Aids are linked with dementia.

But the fact that Dr Wank’s patients were more likely to be infected doesn’t prove that the microbe is causing their disorder. What has put Dr Wank ahead of other researchers in this field is that he appears to have shown that if the infection is treated, the symptoms of the disorder improve. “Earlier attempts to fight the infection just with antibiotics failed,” he says. Dr Wank. “But this more precisely targeted approach has been successful.” He is now hoping to carry out a larger double-blind trial.

But schizophrenia is not the only neurological disorder that has been linked with chlamydia. An American researcher believes that a related strain — Chlamydia pneumoniae — might be contributing to Alzheimer’s. Last year Brian Balin, professor of pathology, microbiology and immunology at the Philadelphia College of Osteopathic Medicine, reported that it is much more commonly found in the brains of Alzheimer’s patients.

Meanwhile, Dr Ruth Itzhaki, at the University of Manchester, has been investigating a connection between Alzheimer’s and the Herpes 1 virus — the one usually associated with cold sores on the lips, although it can cause outbreaks elsewhere. She has found signs of the virus in the brains of Alzheimer’s patients, and in a paper earlier this year reported that those who were infected and had the gene ApoE4 were more likely to develop the disease.

An obvious criticism of the idea that bacteria and viruses contribute to Alzheimer’s and schizophrenia is that these infect a substantial proportion of the population without apparently doing any harm. But as Dr Itzhaki points out, that is not uncommon with infectious diseases. “H.pylori, for instance, is firmly linked with ulcers,” she says. “But only about 10 per cent of those infected with the bug actually develop them.” Both Dr Itzhaki and Dr Wank have found that certain gene patterns make it far more likely that the pathogen will have a damaging effect.

This is all very early work, not least because it has proved so hard to get funding, but it does suggest that already established antiviral or antibacterial treatments may have a role to play in the treatment of mental illness.