People are infected with malaria from parasites inside mosquitoes
More cases of the most dangerous type of malaria than ever before are being brought back to the UK from trips abroad, official figures show.
A Health Protection Agency study identified 6,753 cases of falciparum malaria diagnosed in the country between 2002 and 2006.
This is a 30% increase over 15 years, reports the British Medical Journal.
Experts said many of the cases arose from visits to west Africa made by people visiting relatives and friends.
Malaria is spread by mosquitoes carrying the plasmodium parasite, and can take weeks or months to emerge after the bite itself.
Travel to areas where malaria is endemic has increased sharply in the past two decades, from just under 600,000 visits in 1987, to 2.6 million in 2004.
The HPA study looked at the origins of 39,300 cases of malaria diagnosed in the UK between 1987 and 2006, 20,488 of which were in UK residents coming back from trips abroad, rather than travellers from other countries falling ill during a UK visit.
Many were less dangerous forms of the disease, but nearly 25,000 of the 39,000 were caused by the plasmodium falciparum parasite.
This version is far more likely to prove fatal, and there was a total of 183 deaths.
Plasmodium falciparum is most likely to be contracted in sub-Saharan and west African countries, and about half of those contracting the illness were visiting friends and family in countries such as Nigeria and Ghana.
Only two-fifths of the UK travellers had made any effort to prevent the disease by taking prophylaxis drugs before or during their trip, particularly those born in or with family ties to malaria-endemic areas.
Travel medicine advice is freely available at GP surgeries, and specialist travel medicine clinics in the UK's bigger cities.
Professor Peter Chiodini, head of the HPA's Malaria Reference Laboratory, said: "There is a prevailing myth that travellers who were born in a malaria-endemic country such as Africa have some 'natural' immunity to malaria and this is simply not the case.
"Like all other people who go to Africa and Asia they need to make sure they take their anti-malaria drugs and follow the guidelines that are there to protect everyone."
Dr Jane Zuckerman, from the Royal Free and University College Medical School, and director of the World Health Organisation collaborating centre for reference, research, and training in travel medicine, said that it was important to get the message about malaria to all UK travellers.
She said: "It may be possible, in some communities, to use religious leaders to communicate this, as this can be very effective."