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Statistically calculating the chances of infection

Nov 23, 1998

I recently contracted NSU from an act of unprotected sex in Malaysia. I don't think that the girl was a sex worker or drug addict as no money was exchanged or requested. According to the WHO there are approx 13,000 HIV positive women out of a population of 5,000,000 or .2% of the female population. We had unprotected vaginal sex twice.

If not for the NSU I would have been very worried but not totally panic striken as I now am. How common is NSU in the average adult female population (in the US for example or better yet in developing countries)? Because I contracted NSU wouldn't it mean that the odds of contracting HIV would be 10 fold, 50 fold or even a hundred times more likely than the .2% average? Is contracting NSU almost always because the woman was exposed to unsafe sex at some point or could it just be a "female infection" and not related to her sexual behavior? Finally, does the infection she gave me possibly act as a carrier or catalyist in helping to infect me and thus raise the odds of my contracting HIV if she was in fact HIV positive?

Thanks so much for your input. I have tried over and over to "calculate" my risks of having already acquiring HIV. I will certainly get tested in 3 months and again in 6 months however you could help me sleep again in the mean time by helping me estimate the liklyhood of having already being infected with HIV. I know you probably dont want to assign odds to my situation however some statistical estimation would really help me with my acute anxiety. I know I'm at risk but am I too paranoid???


Response from Mr. Sowadsky

Thank you for your question. I have seen many people trying to use statistical data to determine their individual risk for HIV and other sexually transmitted diseases (STDs). Unfortunately, in real-life situations, no statistic that you see (regardless of the disease), can be used to accurately determine your personal risk of infection. This is for several reasons:

1) Some statistics are only estimates and may not reliably indicate the true incidence of a disease. For example, we do not know the true number of cases of Nonspecific Urethritis/Non-Gonococcal Urethritis (NSU/NGU) in the United States (or in the world for that matter) since this is not a reportable disease in most places. It has been estimated that the number of cases of urethritis in the USA is approximately 1.2 million per year, but it is impossible to know how accurate this estimated statistic really is. Some data suggests that the number of cases of NSU/NGU in the United States has remained relatively stable in the past 30 years (with a possible downward trend in the 1990s). However, since NSU/NGU is not a reportable disease, we do not really know how common NSU/NGU truly is. The same can be said of NSU/NGU anywhere in the world. In addition, statistics for HIV and many other STDs in many parts of the world (especially developing nations) are also estimates, and may not give an accurate count of the true number of cases. And even for diseases that are reportable (for example syphilis and gonorrhea) we know that some cases never get reported, so some statistics underestimate the true number of cases.

2) There are numerous variables that can determine whether a person gets infected with an infectious disease or not. Some sexual activities are higher risk for diseases than others (depending on the disease and the activity). For example, there are many variables that can determine whether a person gets infected with HIV or not. Even luck itself plays a role. It is never a guarantee that you will get any infectious disease from another person. But the more times you are exposed, the greater the chance of infection. No statistical analysis can take into account every variable that determines whether a person gets an infectious disease or not.

3) The level of risk varies tremendously from one individual to another. Some people are at very low risk for HIV/STDs (for example people who use condoms every time and use them correctly). Other people are at very high risk for HIV/STDs (for example people with multiple partners who never use condoms). Since there is a lot of variability of risk from one person to another, any statistic you see cannot be applied to individuals.

If you are trying to statistically determine your personal risk of infection, do not bother looking at statistical information, and put away your calculator. No statistic you find can accurately be used to calculate your personal risk. Again, no statistic can take into account all the variables in life that determine the risk for any infectious disease.

If statistics cannot determine your personal risk of infection, then what are they good for? Statistics, interpreted carefully and with caution, can be used to monitor trends in diseases, especially among large numbers of people. For example, some statistics can indirectly suggest to us whether people are practicing safer sex more often or not. Statistics can tell us trends in diseases over time, which is important in health planning and intervention. Rather than spending your time statistically calculating your risk of infection, if you have put yourself at a realistic risk of infection (based on what you read on this website), get tested. Testing is the only reliable way to know whether you became infected or not. Trying to statistically calculate your risks will not reliably tell you whether you are infected or not. In fact, trying to calculate your risks will only stress yourself out even more. If you find yourself getting stressed out over your fears of HIV or other STDs, consider talking to a mental health counselor to help you cope with your fears and anxieties. This is especially true if your fears and anxieties are interfering with your day-to-day life on an ongoing basis.

In summary, there are a lot of things that statistics can do, and there are a lot of things that statistics cannot do. Calculating your personal risk of infection is one of the things that statistics cannot do. If you have put yourself at a realistic risk of infection, testing is the only way to accurately know whether you became infected or not.

If you have any further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide).

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