Dumb question: The asymptomatic carriers for Coronavirus.

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4. Could be that the initial viral exposure was low

That may give the immune system enough time to respond without going into overdrive and unleashing obvious symptoms.

But some people just don't react symptomatically to certain illnesses, such as Typhoid Mary, but many people are asymptomatic spreaders or the flu or common cold. Others just ignore their mild symptoms and don't ever realize they're sick.