Public health experts think they’ve solved the mystery behind the illness that’s linked to more than 50 deaths of children across Cambodia since April: a virus that causes a common illness in children called hand, foot, and mouth disease. The particular strain of the virus — Enterovirus Type 71 — that destroyed the lungs and led to brain swelling in the children who died, has posed problems in other Asian countries, but it isn’t seen often in the United States and tends to be less dangerous when it infects US children. “We’re not sure why that is,” said Dr. Al DeMaria, state epidemiologist at the Mass. Department of Public Health. “It may be a slightly different strain that comes here or have something to do with a difference in hygiene practices or overall health of our population.” More commonly, though, the strains of virus that cause hand, foot, and mouth disease in this country are relatively benign, usually infecting children under age 5 and causing fever, crankiness, and blister-like growths in the mouth as well as a rash on the hands and feet. Dehydration is the biggest concern, so parents need to make sure infected children drink enough. Kids who find it too painful to swallow may need to be admitted to the hospital to get IV fluids, said Dr. Thomas Sandora, an infectious disease physician at Boston Children’s Hospital.