It's fine with "is".This is an article from BBC News.
BBC News - Coronavirus: Sars-like virus death reported in UK
Coronavirus: Sars-like virus death reported in UK
A patient infected with a new respiratory illness similar to the deadly Sars virus has died in the UK.
The exact source of the new virus and how it spreads is still unknown. The leading theory is that it comes from animals and the new Sars-like virus does appear to be closely related to a virus in bats.
I have a question about the blue sentence. Shouldn't it be "are" instead of "is"?
I think the sentence says that two things ("the exact sources of the new virus" and "how it spreads") are unknown.
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