When a species of mosquito normally resident in North Africa was found living on Anglesey, experts were astonished. The species, Anopheles algeriencis, was detected in 1988 and had survived in the hotter summers and warmer winters of recent years. Concerns are growing that climate changes could open the door to an invasion of tropical diseases.

Some of these concerns have been expressed in an unpublished report prepared by the Government's Public Health Laboratory Service two years ago. In July Greenpeace published its own report, listing illnesses ranging from malaria to bubonic plague and tick-borne infections, which enter the body through the skin.

One of the authors of the PHLS document, Dr Ian Burgess, deputy director of the Medical Entomology Service at Cambridge University, says: 'Malaria was with us until between the wars and was fairly prevalent among people living and working in rural parts. The strain we had here was not nearly as serious as the type that kills people in the tropics, but it is a very nasty illness.

'There is no evidence that the mosquitoes in North Wales are infected with malaria, but the possibilities of a link exist because malaria mosquitoes are discovered here, on planes and ships.'

The last major epidemic of malaria in Britain came just after the First World War, when thousands were affected. It lingered for many years in parts of southern England and finally died out in the Fifties. Malaria is recorded in our history. Cromwell is said to have died from it, and Sir Walter Ralegh's apparent nervous shaking as he was about to be beheaded was put down to the disease.

Dr Burgess was disappointed that the PHLS did not publish the report. 'Everyone was under the impression that the findings could be made public and I think the public are entitled to know what the potential risks may be.'

Dr Chris Curtis, an entomologist at the London School of Hygiene and Tropical Medicine, believes the British strain of the disease, Plasmodium vivax, could make a comeback, but he does not expect to see many cases. 'The last time we had any reported incidences were in the Fifties,' he says.

But Dr Burgess thinks that a greater threat comes from a range of tick-borne diseases spreading up through southern and northern Europe. Leishmaniasis, endemic in France, is a parasitic disease that leads to multiple ulcerated lesions which can last for six to 12 months, although relapsing forms might last for 50 years. Another form of the disease is characterised by long, irregular bouts of fever, enlargement of the spleen and progressive emaciation. If untreated, it can lead to death. It is passed on by a species of sandfly, and climatic factors are decisive in its spread. Even a small degree of warming could lead to an extension northwards of the disease, possibly into southern England.

Rickettsiosis, an illness which can be spread by dog ticks, is also marching north and could arrive via holidaymakers. In its severe form it can cause high fever, delirium, stupor and coma.

'These types of infection could be much more easily spread and more difficult to eradicate than an outbreak of malaria,' says Dr Burgess. 'We really have to be on our guard to detect any invasion of these ticks.'

One which is creating problems in Europe at present is a disease spread by sheep ticks called viral encephalitis. It causes severe headaches, fever, nausea and photophobia and can lead to a form of meningitis. Mortality rates are high and it is already well established as close as eastern France.

Dr Burgess says that warming is also likely to leads to outbreaks of dysentery, paratyphoid and water- borne parasitic diseases. 'Dysentery outbreaks at schools are becoming quite common. In recent years there have been more than 5,000 reported cases.'

He pointed out that in 1989 in Hull, 477 people were affected with a diarrhoeal illness when a parasite got through after the water filter system was bypassed. This was reported by the departments of the Environment and Health in their investigation into the outbreak. One particularly virulent infection caused by a water parasite, cryptosporidiosis, can survive even after chlorination and there have been several thousand cases.

'We cannot be blase. In the past we were vigilant about diseases like smallpox and we need to be aware of new illnesses so they don't gain a foothold,' Dr Burgess warns.

'Potential Impact of Climate Change on Health in the UK', pounds 2.50 from Greenpeace, Canonbury Villas, London N1 2PN.