Thanks, Marius. I understand it is the best to put the meaning the dictiorany says on this sentence. I think, in medical terms, circulating viruses means ones that scientists anticipate circulating after the outbreak is observed.
I totally agree with you, Marius. I think my point is now the same as yours. I used the word outbreak, which I just meant 'viruses are observed.' I'm not quite sure what outbreak means.
Thanks so much, Marius. I usually take Cambridge and ALC , but if I took M-W, which I should, I would've understood it earlier. In other words, if you hadn't told me I'd be struggling now.