it does happen with some viruses
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Not just HIV. Polio is a good example where the vast majority who get it aren't symptomatic (or barely) yet can & do spread the disease. The live weakened Polio Vaccine takes advantage of this to infect & effectively immunize many of those who come in contact with the vaccinated.
Children also notoriously spread diseases that don't affect them. "Asymptomatic spread" is an assumption based on what containment measures work to what extent; you only get ebola or MERS if you're in the room with someone who's severely ill, and thermometers work to catch SARS.
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Everything is a symptom of C19, including bizarre subjective effects like loss of taste and smell. If you want to narrow it to a specific objectively measurable symptom - fever, say - well, thermal screening didn't protect the White House.
End of conversation
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