According to HPA
In 2007,    The prevalence of HIV (both diagnosed and undiagnosed infections) among MSM aged 15-44 was estimated to be 8.5% (range 7.0-10.5%)

By 2007, 32% of all MSM seen for HIV care were aged over 45, and 17% were over 50.
http://www.hpa.org.uk/web/HPAweb&HPAwebStandard/HPAweb_C/1203928687610

So well over 10% of MSMs are infected.
Probably 12% to 15% or even 20%
For comparison, the great flu pandemic of 1918 killed 3% of population, and the Great Plague of 1665 killed 15%