Four such different (novel) virus subtypes have appeared in humans since the global epidemic (pandemic) of 1918, which killed tens of millions of people.
But antibodies against flu viruses you've encountered in the past can't protect you from new influenza subtypes that are very different immunologically from what you had before.
In a recent example, they have built a big data infrastructure capable of cross-referencing the relationships between 17, 000 genes and five major cancer subtypes across 20 million biomedical publication abstracts.
The CDC said it doesn't have complete data on subtypes, but of the data it does have, the proportion of children diagnosed with each of the subtypes is similar in 2006 and 2008.