Luck and statistics are really important.

One person might become HIV positive after one exposure. Another person might have many exposures over years and still not become HIV positive.

In many circumstances we cannot explain this. Life is not always fair.

It might be that the risks that are difficult to measure are higher when infections occur. When they don’t occur, the same difficult-to-measure risk factors may all be lower.

As these can’t be measured or changed, this gets put down to good or bad luck.

Some researchers also emphasise the role of statistics.

Even when one partner is HIV positive and another is HIV negative, the risk from not using a condom one time, might be 1 in 500 (0.2%).

This will be a bit higher for anal sex than vaginal sex, and a bit lower for insertive compared to receptive sex (but remember circumcision). In general these are low single risks.

But new infections occur because it only takes one exposure to transmit HIV.

So if 500 people have sex without a condom, one person might become positive. But if 500,000 people do the same thing, then 1000 people would become HIV positive.

Luck, or chance, or unmeasurable factors are related to time and the number of exposures.

Statistically, most people will be lucky once, but the chance of being lucky 10, or 100 or 500 times gets increasingly slim.

For some who takes 50 risks, the previous example with odds of 0.2% increases the risk to 9.5%.

This figure comes from mutiplying the risk of not becoming positive 50 times and subtracting the result from 100%. i.e. (i) 0.98 x 0.98 x 0.98 etc (50 times) = 0.98 to the power of 50 = 90.47; (ii) 100 minus 90.47 = 9.53%). This assumes that each risk is the same every time.

1 June 2016