In the years that followed, scores of the couple’s friends died of AIDS but he never got ill, despite being as sexually active as them all and not taking any special precautions.
When he realized he was different, he volunteered to work with doctors to find out why.
‘I couldn’t infect the CD4 cells,’ Dr Bill Paxton, who the worked at the Aaron Diamond AIDS Research Center in New York, said. ‘I’d never seen that before.’
Years later, researchers isolated the reason. H.I.V. gets into the white blood cells by fitting into two receptors but Mr Crohn’s second receptor was flawed due to a genetic defect.
The anomaly, found in less that 1 per cent of the population, saved Mr Crohn’s life.
Of course, being childless, this particular instance of this particular set of genes isn’t going to perpetrate down the generations. Bit it’s an interesting example of quite how deep that gene pool is. Quite how difficult it would be for any one disease to kill us all.
The other example I can think of is the way that the Black Death significantly changed the genetic make up of Europe. And of course this is very much Matt Ridley territory, all Red Queen and so on.
And, at the extreme, it explains the existence of sex: which brings us rather full circle giving that we’re talking about HIV.