Then there were the night sweats. He would wake to find himself and the bedclothes wringing wet. And every joint, in his knees, hips and shoulders, was painful. "I thought it was all a bit odd but I kept telling myself it would go away soon, that it was a virus or something. I felt the way you do the day after a really hard game of rugby but I hadn't been playing rugby."
In retrospect, he says he was denying that anything was wrong. A third- year medical student at the Royal Free Hospital, London, Rob Poulter, 24, had indulged in a little self-diagnosis and was not optimistic: his symptoms could have meant cancer or even Aids, and he didn't really want to know which.
But that night at the theatre was a turning point. He remembered an Australian girl he'd met at a hostel in Durban at the end of the holiday in August last year. He'd complained of feeling run-down and "fluey". She'd asked him where he'd been and as he reeled off the places he and three friends had taken in on their southern African tour, she said: "Lake Malawi? You went to Lake Malawi? Oh, then you've probably got bilharzia. I've had it and all my friends have had it." She recommended a course of antibiotics, costing about pounds 40 from a local pharmacist. He ignored that advice.
Bilharzia, or schistosomiasis, is a parasitic infection caused by a tiny flatworm that bores through moist skin deep into human flesh. The worm, endemic in Africa, lives part of its life cycle in freshwater snails, which excrete it in their faeces into water where unsuspecting humans wash or bathe.
One of the most common illnesses worldwide, it infects more than 200 million people and kills 200,000 annually. But a mini-epidemic among young visitors to Africa - particularly Lake Malawi, one of the continent's most famous beauty spots - is alarming British doctors. One leading infectious disease expert, jokingly introducing a lecture on schistosomiasis in London recently, began with: "This is the disease caught by backpackers visiting Lake Malawi."
The number of cases reported to the Communicable Disease Surveillance Centre in north London has risen since the mid-Eighties. But the infection is reported only voluntarily. Provisional cases reported for 1994 for England and Wales are 204, compared with a low of 54 in 1988.
Dr Nick Beeching, senior lecturer in infectious diseases at the Liverpool School of Tropical Medicine, says about half of the cases he has seen are traceable to Lake Malawi. In one group of 18 students on a scuba-diving holiday, 15 were infected.
Rob Crossley, 22, who camped with Rob Poulter at the Cape Clear resort on the lake shores, also became ill with a less severe form of the infection. "For 10 days we washed, swam, and dived in the lake. We used lake water to cook with. It was as clear as a bell.
"You could see snails in the water, and I did think 'hm, bilharzia?' but I thought it was safe. No one mentioned it and there was nothing in the leaflet from the Malawi Tourist Board."
He knows of at least 10 people who returned with the infection.
Infectious disease experts say the Malawi government is to blame. For years it has insisted that the lake is free of schistosomiasis, so anxious is it to preserve its reputation - and the flow of tourists' dollars. It is only now that officials are admitting that there may be a problem in "some areas". They insist that all the popular tourists spots - Nkopola Lodge, Mlangeni Lodge and Club Makokola - are clear, thanks to periodic snail eradication programmes, contrary to travellers' reports. "It is the tourists who swim in other, prohibited areas who are putting themselves at risk," a spokeswoman at the Malawi High Commission in London says.
Africa on a Shoestring, a low-budget travel guide beloved of students, is also culpable. Until recently, the guide, which is published by Lonely Planet, reported the lake as safe to swim in. Last summer the publishers were forced to print 30,000 erratum slips. The latest edition warns against swimming in the lake but, unfortunately, travellers don't always have the latest editions. The books can remain in circulation for years.
Treatment of schistosomiasis is a short-course of a fast-acting antibiotic, praziquantel, which kills the worms. However, symptoms vary widely and can take up to three months to develop. Some GPs miss the diagnosis of schistosomiasis, even when told their patient has recently returned from Africa.
Dr Peter Chiodini, a consultant parasitologist at the Hospital for Tropical Diseases in north London, says there needs to be better information for travellers and doctors.
"There is a lot of confusuion. Is the lake infected or not? There is no doubt that more travellers are being exposed. We are definitely seeing more urinary cases here."
Rob Poulter was not quite so lucky. He developed a much rarer form of bilharzia than the bladder infection that causes bloody urine. The parasite had migrated to his lungs. When he presented himself at the Hospital for Tropical Diseases in October last year, he was admitted immediately. He had a temperature of 102, and a lung X-ray with an alarming "snowflake" pattern of diffuse shadows where the lymph glands around the bronchus were swollen and inflamed.
When someone returns from tropical regions with feverish symptoms doctors normally regard it as "malaria until proven otherwise". Bilharzia was an obvious possibility in Rob's case given the Lake Malawi connection - however, the lung X-ray was far from typical. Rob's worst fears were realised when he was first told that he may have Hodgkin's Lymphoma, a type of cancer of the blood, or an auto-immune disease affecting the lungs, known as sarcoidosis.
Then his blood and urine tests came back positive for schistosomiasis. A dual diagnosis of bilharzia and sarcoidosis was given and he was referred to a chest specialist. The specialist sympathised with the student: "You've been very unlucky and picked up both of these at the same time," he said.
Rob didn't believe him. As soon as he started on the antibiotic treatment for bilharzia, his health improved markedly. His breathing became easier and the fatigue he had been suffering left him. But the night sweats eased only a little and the joint pain persisted. He had a second course of the drug in May and says that appears to have resolved the problem.
He says his over-riding emotion is one of annoyance that he wasn't warned about the risks of schistosomiasis. But he is realistic about the allure of Lake Malawi for travellers, he says. "One of the doctors who treated me went the following year. He knew all the risks but could not resist stripping off and diving in. He came back with bilharzia."