When Dr. Thumbi Mwangi was a child growing up in Kenya, his father would send him out to care for the calves.There was a disease — East Coast fever — infecting cattle. If the cattle were infected with the protozoan, it could be deadly. Mwangi’s job was to rub the young cattle gently behind the ears, to see if he could feel their lymph nodes. If the lymphs were swollen, it could mean that the calf was getting sick.