Early tests of a malaria vaccine have shown that it is more than 50% effective at preventing the deadly disease in infants and toddlers. Malaria is a tropical disease caused by a parasite(寄生虫)that is spread to humans through the bite of an infected mosquito. The parasite travels quickly to the liver, where it replicates(自己複製する)before moving into the bloodstream and attacking red blood cells. The vaccine is designed to attack the parasite before it can infect the liver. This is the first malaria vaccine to make it this far in the testing process, and a larger and longer running test is slated(予定である)to begin early next year.