It may seem as if flu viruses get chosen based on a sluggish and never-ending game of bingo.

H1N1? … Does anyone have H1N1?

The next number is… H7N9. Please check your cards, the number is H7N9….

H5N1… do we have a bingo? Please bring your card in to be validated. The prizes tonight include congestion, fever, and sore throat.

Who comes up with these designations and what do they mean? As is often (but not always) the case in science, there is a method to this apparent madness.

The illness we commonly refer to as “the flu” is caused by a series of viruses called influenza viruses.

Let’s stop here. What is a virus? Beyond this hazy picture of an invisible microbe that causes ill health, a virus must fulfill the following conditions:

• It is a parasite that infects a host and makes more of itself inside the cells…