1. Estimated new HIV infections by transmission category, extended back calculation model, 50 US states and the District of Columbia, 1977 – 2006 Source : Hall et al. (2008a). 80 000 70 000 60 000 50 000 40 000 30 000 20 000 10 000 0 1977- 1979 1980- 1981 1982- 1983 1984- 1985 1986- 1987 1988- 1990 1991- 1993 1994- 1996 1997- 1999 2000- 2002 2003- 2006 Infections Period Men who have sex with men (MSM) Injecting drug use (IDU) MSM / IDU Heterosexual Figure 23 Tick marks denote the beginning and end of a year. The model specified periods within which the number of HIV infections was assumed to be approximately constant.