today. The strain is currently used in the trivalent (measles, mumps and rubella) MMR vaccine that he also developed, the first vaccine ever approved incorporating multiple live virus strains. Like many other vaccines and medications of that time period, the vaccine was initially tested in children with intellectual disabilities who lived in group homes; this was because, given the poor hygiene and cramped quarters of their accommodations, they were at much higher risk of infectious disease. Hepatitis B vaccine He and his group invented a vaccine for hepatitis B by treating blood serum with pepsin, urea and formaldehyde. This was licensed in 1981, but withdrawn in 1986 in the United States when it was replaced by a vaccine that was produced in yeast. The latter vaccine is still in use today. By 2003, 150 countries were using it and the incidence of the disease in the United States in young people had decreased by 95%. Hilleman considered his work on this vaccine to be his single greatest achievement. Liver transplant pioneer Thomas Starzl said "...controlling the hepatitis B virus scourge ranks as one of the most outstanding contributions to human health of the twentieth century...Maurice removed one of the most important obstacles to the field of organ transplantation". Later work and life In his later life, Hilleman was an adviser to the World Health Organization. He retired as senior vice president of the Merck Research Labs in 1984 at the mandatory retirement age of 65. He then directed the newly created Merck Institute for Vaccinology where he worked for the next twenty years until his death in 2005. At the time of his death in Philadelphia on April 11, 2005, at the age of 85, Hilleman was Adjunct Professor of Pediatrics at the University of Pennsylvania in Philadelphia. Method and personality Hilleman was a forceful man yet at the same time, modest in his claims. None of his vaccines or discoveries are named after him. He ran his laboratory like a military unit, and he was the one in command. For a time, he kept a row of "shrunken heads" (actually fakes made by one of his children) in his office as trophies that represented each of his fired employees. He used profanity and tirades freely to drive his arguments home, and once, famously, refused to attend a mandatory "charm school" course intended to make Merck middle managers more civil.