HIV-1; An interesting problem for our generation

Human immunodeficiency virus (HIV) is an interesting virus for several reasons. As you probably know, this is the virus that leads to "full-blown" acquired immune deficiency syndrome (AIDS).

To give a brief, unreferenced history of HIV/AIDS (although most can be found in Levy's "HIV and the Pathogenesis of AIDS (2007)):

In the early 1980's gay men in New York and Los Angeles fell very ill as their immune systems seemed to just "collapse". In 1982 it was formally labeled AIDS. It was not until a few years later (~1984) that the virus was "discovered" and named HIV (its a bit more complicated than this but this is an overview!).

It turns out that there is a serum sample positive for HIV from the Congo that dates to 1959. In 1976 3 Norwegians were reported as having HIV-1 (when I say "HIV" I generally am referring to all classes of "HIV-1"; only ~100 cases of HIV-2 exist in the U.S.).

The unique thing about the HIV is that it is a "retrovirus". What this means is that when the RNA virus gets into a cell, it makes a "complementary" DNA strand, and then makes a "complementary" DNA strand for the new strand. Now a full double helix of virus exists. This is then inserted into the cells genome. The problem is that this process of compiling DNA is extremely error-prone, accounting for up to 10 base pair changes in HIV genome per replication cycle.

I digress. It is actually quite difficult to "catch" HIV during most exposures. In fact, the likelihood after a single exposure to the virus varies depending on what that exposure is. The following numbers are from Levy's 2007 book:

Data are presented in the following format:

Exposure Type (likelihood of infection with single exposure, %); [Global Total,%]
Sexual interecourse (0-1.0%); [70-80%]
Injecting drug use (0.5-1.0"%); [5-10%]
Maternal transmission, pregnancy and breast milk (12-50%); [5-10%]
Blood transfusion (>90%); [3-5%]
Health care work, needle sticks, etc (0.1-1.0); [<0.01%]

You might find these numbers quite surprising! However, most people engaging in at-risk behavior do NOT do it one time (think - sex and drug use are usually behaviors). Further, transmission depends on exchange of fluid (blood, sexual fluid, pregnancy) AND that the virus be in the fluid (mainly via infected cells) AND that the virus is able to get into the "victim's" cells and infect them.

Of course, using these broad ranges for chance infection, we can easily use probability theory to determine the % upon repeat exposures. For example, let's consider having unprotected sex with a woman who has HIV-1. Let's pretend that you have sex 10 times with her. If we arbitrarily pick 0.5 as % upon one exposure (median of 0-1.0), this means that 99.5% of the time we will not get infected.

Thus, to determine probability of "not being infected" after 10 sexual encounters: (0.995)^5

= 0.951

Put another way, there is a 5% chance you will contract HIV after 10 sexual exposures.

Finally, I should point out that most HIV transmission is via heterosexual contact, not homosexual contact, although any activity per se that tears the lining of the rectum or vagina increases the likelihood of transmission.