"Single cells bear variable combinations of these NK cell receptors (NKR), presumably resulting from
stochastic gene activation/silencing events that take place during their maturation. The diversity of NKR observed in different individuals is in part genetically determined, as distinct KIR haplotypes include variable sets of genes. On the other hand, there is evidence that
microbial infections may also influence the NKR repertoire. In this regard, murine cytomegalovirus (MCMV) promotes an expansion of NK cells bearing the Ly49H receptor specific for the m157 viral glycoprotein, that play a crucial role in the immune response to infection."
The point is not that everyone born in hospital has asthma - it is that everyone with asthma was born in hospital.