Besides being diverse, bats are very abundant and very social. Many kinds roost in huge aggregations that can include millions of individuals at close quarters. They are also a very old lineage, having evolved to roughly their present form about 50 million years ago. Their ancientness provides scope for a long history of associations between viruses and bats, and those intimate associations may have contributed to viral diversity. When a bat lineage split into two new species, their passenger viruses may have split with them, yielding more kinds of virus as well as more kinds of bat.
