Coronavirus is 30,000 RNA 'letters' long, meaning there are over a quintillion possible genome permutations.
Variants of the original SARS-CoV-2 are now in wide circulation. That means the third wave of COVID-19 has come with new questions about the variants, their effects and what might come next.
The US lags in testing coronavirus samples from COVID-19 patients, which can help track the spread of the virus and the emergence of new variants. But labs are ramping up this crucial surveillance.
Immunity fades rapidly for some infections (influenza) but lasts a lifetime with others (measles). Here's what we know about COVID.
The real concern is if two variants infect the same cell and swap genetic material.