Blood tests confirmed that Ms Hadbury had falciparum malaria, the most severe form of the illness. She was immediately admitted to the Hospital for Tropical Diseases, London, by which time she had high fever, extreme nausea and tinnitus. "I couldn't lift my head off the pillow," she recalls.
She was treated by intravenous infusion with the anti-malarial drug quinine; one month after being discharged she is now fully recovered. Last year, 13 other people in the UK were not so lucky. They died because they had not taken the right preventive treatment; nor had they sought attention quickly enough when symptoms appeared.
Many specialists now fear that the incidence of malaria in Britain and deaths from the disease are set to rise. This is because mefloquine, one of the most effective drugs to be developed in the fight against the disease, is no longer available on NHS prescription. The costs of taking it as a preventive measure have increased more than fivefold since February.
Like other anti-malarial drugs, mefloquine has to be taken for several weeks before and after visiting a malarious region. It is usually recommended for travellers to high-risk areas such as sub-Saharan Africa, parts of Asia and Latin America, where malarial parasites have become resistant to older drugs.
Until March this year, when the Government decided it should be paid for privately, mefloquine was available on NHS prescription for pounds 5.25. Overnight the cost went up to nearly pounds 30: about pounds 22 for eight tablets - enough for one person for three weeks - plus a private prescription fee to a GP for an additional pounds 6.50. For a family of four, the additional costs of taking the drug are pounds 110. Ms Hadbury did not take mefloquine because she has an adverse reaction to it. But doctors fear many other travellers are now not bothering because of the cost involved, opting for cheaper, less effective preparations or none at all.
"More people may choose not to add to the cost of their holiday and go without," warns Dr Ronald Behrens, consultant in tropical and travel medicine at the Hospital for Tropical Diseases, London.
Pharmacists such as Boots, and leading manufacturers including Roche, which produces mefloquine, say it is too early to estimate the effect on sales of taking the drug off prescription.
Dr Behrens has estimated that the use of mefloquine prevents 11 deaths and 3,000 cases of malaria a year. He also calculates that it saves the NHS pounds 22m annually on treating malaria cases.
Malaria has become more common in Britain with the rise in long-haul travel: about 800,000 travellers now take holidays in malarious regions and about 2,300 return with malaria every year. The illness, especially the falciparum type, can produce fatal complications affecting the kidneys, liver, brain and blood.
Many of the drugs developed both to treat and prevent the disease are related to quinine, first used to combat malaria in the 16th century. They work by killing the parasites in the blood. Two are available over the counter: proguanil, which costs about pounds 10 for 100 tablets, and chloroquine, at about pounds 2 for 20. Different drugs are recommended by doctors depending on the risk of exposure and the extent of drug resistance.
In many parts of of the tropics, malarial parasites have become resistant to chloroquine. Mefloquine is a newer drug that acts against all the malaria parasites and is considered more effective than other products.
Whereas mefloquine now has to be paid for, the four most commonly needed vaccines for travellers - cholera, typhoid, hepatitis A, and polio - are still available on NHS prescription. But according to a Department of Health spokesman, whether the NHS should fund the cost of travel vaccines is under discussion: "At the moment there is an internal review of immunisation policy within the department, but any kind of proposals or options would have to be first considered at ministerial levels."
Some doctors say they understand the Government's viewpoint. Dr Chris Ellis, consultant physician in the department of infections and tropical medicine at Heartlands Hospital, Birmingham, says: "With an NHS strapped for cash, a view could be taken that it should not provide care for people who choose to go overseas and expose themselves to risks not encountered here."
On the other hand, he points out, "if the costs of treating people with malaria increases to a level greater than the cost of providing drugs, then you can forget the philosophy and it's down to economics".
Simon Hughes, the Liberal Democrat MP whose brother Richard died of malaria three years ago after returning from his honeymoon in Kenya, says Richard had taken anti-malaria tablets but still contracted the disease. The symptoms were diagnosed as flu until it was too late.
He argues that taking the drug off NHS prescription means many people will no longer view it as a medical necessity.
"A lot of people who are at risk travel because family members are on another continent. They are not necessarily on a high income, they don't regard it as a luxury holiday and seek to do it on as low a cost as possible," he points out.
Clare Hadbury is horrified that mefloquine is no longer available on NHS prescription. A health educator who works a lot in Africa, she says: "I know how difficult it is to persuade people to take anti-malaria drugs. Taking it off prescription is just another deterrent."
The Hospital for Tropical Diseases Health line (0839 337733) has information on the malaria status of individual countries.
The London School of Hygiene and Tropical Medicine has a malaria 24-hour advice line: 0891 600350.
The Department of Health publishes a Health Advice for Travellers leaflet (T5),available free from main post offices. It gives a country-by-country guide to the protection needed.
Malaria: the facts
Malaria is prevalent throughout the tropics, affecting up to 300 million people worldwide every year. It is transmitted to humans by the bite of the female anopheles mosquito, which needs a diet of blood for its eggs to mature.
Malarial parasites then infect the red blood cells, breaking down the haemoglobin. Symptoms, which include chills, fever, headache and fatigue, usually strike a week or two after the bite but may take as long as a year to appear. Malaria can cause damage to the kidneys, spleen, liver and brain, resulting in coma.
Four species of malarial parasites cause disease in humans. The most serious is falciparum, which can be fatal within a few days of symptoms appearing. Less dangerous are vivax, ovale and malariae.
Antimalarial tablets need to be taken for four weeks after travellers return to Britain. Even taking antimalarial drugs does not protect 100 per cent against the disease. Personal protection is needed against bites, including nets, insect repellent and adequate clothing.
Anyone who has flu-like symptoms within a year of returning from a malarious region should seek medical help immediately.