Scientists who study viruses say they don't know what a pandemic strain would look like.
pandemic = in medicine, an epidemic that infects a population over a very large area, such as an entire country or continent.
strain = in this context, a variation on an organism that is distinctive but not necessarily a different type or breed of that organism.
I think the example is saying that scientists don't know how they would identify a version of a virus that might have the potential to infect a population over a very large area.
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