QUESTION 1 . An epidemiological investigation was begun on July 1, 2000 among a population of 1,000 individuals. Three individuals were found to have leukemia on July 1st. During the ten-year follow-up period, five new cases of leukemia were diagnosed. Among the eight leukemia cases, four deaths occurred during the ten-year follow-up period. Two additional individuals, neither of whom had leukemia, were lost to follow-up at some point during follow-up. None of the remaining 990 study participants became ill, died, or were lost to follow-up during the follow-up period . The diagram below indicates the experience of the 10 study participants, using the following symbols: :case recognition (diagnosis of leukemia) :onset of leukemia was prior to start of study period - --- :observation period during follow-up X :status at termination of observation (i.e., alive, dead, lost) case 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 alive --X alive F-X dead lost dead - 3-- F-X alive dead --X alive lost 10 dead What was the prevalence of leukemia on January 1, 2005