Some genetic mutations make some things harder to catch. IE, the same gene that makes me more likely to have arthritis also makes me more immune to influenza and HIV. Now....i have had flu type illnesses, I just don't get them very often (about every 7-10 years I'll come down with a bad flu of some sort). So, assuming that the coof is a real thing, one can also assume that some people just don't catch it the common strains that other people get.