When people from non-endemic areas, who have little natural immunity, travel to endemic ones, they are especially vulnerable.

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However, since natural immunity to malaria exists, researchers are reasonably optimistic that a vaccine, which stimulates this immunity artificially, should be feasible.

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Director of the Public Health Agency Dr Caroline Harper said this age group was vaccinated last year and now 50-60% had built up some level of natural immunity.

BBC: Health Committee

When the flu spreads person to person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight, because people have no natural immunity.

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In 1957, when flu swept through Hong Kong, Mr Hilleman identified the virus as a new form to which people had no natural immunity and passed on his findings to vaccine-makers.

ECONOMIST: Obituary

Some very cool science is on the drawing board: a recombinant vaccine that pairs salmonella with a gene from, say, the organism that causes tuberculosis as an oral delivery system to stimulate the body's natural immunity to both bacteria.

FORBES: Yes, there is a pulse