When public health plots out the course of an epidemic, several points are taken into consideration.
How easily can the disease be spread? Is it airborne, transmitted through contaminated food, or passed along though more intimate activities?
How long is it transmissible in one individual? Is it something like the Ebola virus that kills the host quickly before he or she can infect others, or is it something like the flu which spreads in the first few days of infection while the person is still going about their daily routine before they get laid up in bed--and thus become less likely to infect others?
In the case of HIV, the long latency period where someone can be infectious but still appears to be healthy can last for years--which increases transmission, while the relative difficulty of transmission (one has to be a needle-sharing or unprotected sex partner) slows the spread.
Early in the course of an epidemic the most vulnerable become infected, and as increasing numbers of infected people mix with those at risk, the likelihood of an individual coming into contact with a "carrier" increases. Eventually as a population becomes saturated, the prevalence--the percentages of those infected--levels off, and the number of new infections (incidence) becomes equal to the number of people who die or recover from the disease.
We have seen this "S" curve in HIV among gay and bisexual gay men, where one in four are positive in most big US cities. Often when we see a rapid rise in new cases, we are witnessing the spread of HIV among an new at-risk population.
Most recently we've seen a rapid rise of HIV among young black gay and bisexual men. When we drill down in the data from these and other studies, we see that condom use and transmission knowledge are high among this group. So what gives?
For one, young black gay men today are more likely to have sex with other black gay men than young black gay men ten or twenty years ago. Because HIV prevalence is higher among African Americans, the choices these young men make today make them more likely to come across a positive partner than in years past. In the 80's and 90's, HIV prevalence was still on the "upswing" of the S-curve, so there were a lot less gay men with HIV overall. Looking at the schematic above, a black gay man in his 50's was having sex in his 20's at T1, while a young black gay man today is having sex at T2.
By helping folks understand how different diseases move through a population, we can let at-risk folks understand their risk is more than the plumbing of sex. It's also about viral loads, risk groups, and things like drugs use and STDs--which can move someone from a relatively low-risk to a high-risk group.