If you've ever suffered from the ailment known as swimmer's itch, you'll be relieved to know that this dread disease is about to get some serious treatment in the scientific worldl. The National Science Foundation has awarded tiny Hope College of Holland, Mich., $15,334 to study the mysterious cycle that sets thousands of persons scratching across the northern two-thirds of the continent. Roughly the disease goes like this: Little fork-tailed parasites called schisosomes enter lakes with the excreta of birds and rodents. Upon hitting the water they thrash about madly in a life-and-death search for one of several types of snails that can serve as hosts. If no suitable host is found in 24 hours, according to Hope biology Prof. Harvey Blankespoor, the little devils die and that's that. But if they find a host, they grow to maturity and reproduce. Larvae from that second generation then reenter the water. When they encounter human flesh, which usually means a swimmer, they try to bore into it but get stuck in the skin and die. When that happens about three persons out of 10 begin to itch like crazy. Why some people are immune and others are not, and why the ailment turns up in some years and not in others are among the mysteries that Blankespoor and his colleagues will probe. He also hopes a way may be found to eliminate the snail hosts without ecological damage.