https://books.google.com/ngrams/graph?content=interferon
This allows it to engage in uncontrolled replication for up to two weeks before the adaptive response brings it under control.
This massive viral load greatly contributes to transmission.
(Or at least SARS does this.)
https://cmr.asm.org/content/20/4/660.long
https://books.google.com/ngrams/graph?content=interferon