Flu is a hereditary illness:
The case Dr Casanova reports is of a then-two-year-old girl admitted to the Necker [Hospital for Sick Children, in Paris] in 2011 with severe flu. He was one of the girl’s doctors, and her symptoms were so extreme (technically, they constituted what is known as acute respiratory distress syndrome, or ARDS), that he suspected there might be something unusual about her. He therefore sequenced her genome and, in so doing, discovered she had two broken copies (one from each parent) of the gene encoding a protein called interferon regulatory factor 7.
This protein, as its name suggests, stimulates production of interferon, an antiviral molecule. Absence of interferon made the cells lining the girl’s respiratory tract more vulnerable to flu infection. It also meant that when such infection happened, her immune system, lacking its first line of defence, unleashed an inflammatory reaction so big that it was itself damaging. This inflammation caused the ARDS.
All very unfortunate for the girl concerned, then—though she survived and remains healthy (regular vaccination keeps her influenza-free). But Dr Casanova thinks her case is of wider significance, for he has now identified 30 other people with genetic faults similar to those of his first patient. He estimates, on this basis, that about one person in 10,000 has impaired interferon production caused by genetic errors.
That may not sound like much but it would, he calculates, be enough to account for the incidence rate of the most severe cases of seasonal influenza (as opposed to the epidemic sort which emerges from time to time). It could also explain accumulating evidence that some entire families seem particularly vulnerable to flu.