If you return from a trip abroad to find you have projectile vomiting, roaring flatulence, sulfurous belching and explosive diarrhea, the bad news is that you won’t die; you just have an attack of giardiasis, a form of purgatory devised by the single-celled parasite known as giardia. Giardia infections can linger for months because the parasite plays a cunning defense against the body’s immune system. In its genomic wardrobe, it has 190 coats to choose from. As soon as the immune system has generated antibodies against one coat, giardia switches to another. Because of the parasite’s persistence and infectivity, some 280 million cases of giardiasis occur in the world each year, the World Health Organization estimates, though most of these are in developing countries where people are more inured to the disease. Giardia’s offensive game could have a fatal weakness, however. Biologists led by Hugo D. Luján at the Catholic University of Córdoba in Argentina have gained a striking insight into its coat-shuffling stratagem. With this knowledge, they have accomplished a cunning counterploy: they have forced the parasite to make and wear all its coat proteins at the same time. This altered parasite, they hope, should serve as the perfect vaccine, because it immunizes the body to the full repertoire of giardia’s coat proteins all at once. The idea has worked well in animal tests, Dr. Luján said.