system
Given the following post, respond as if you're commenting on Reddit:
human
How soon after exposure is the flu contagious?

I know that you are most contagious in the couple of days before symptoms show and for the couple of days after. 
But how does that apply to exposure. For example, a person is exposed in the afternoon, are they automatically contagious after exposure or does that take time to "build up" in the body?
gpt
No, they are not immediately contagious. It could take days to become contagious.

Initially the virus multiplies unchecked and it reaches a peak concentration. This is when you are most contagious. Shortly after reaching peak concentration, you begin feeling symptoms as the immune system has realized it is under attack. Normally by the time you feel symptoms the concentration of the virus in your body has already begun to decrease thanks to the immune response.

This is true of influenza and many other viruses. The time it takes between you getting infected and symptom onset is called the incubation period and it's normally more than 24 hours for influenza.

For Covid-19 the average incubation period is several days, but you could be spreading the virus for 24 hours or a little longer before you first feel symptoms.

You feel fine, but you are highly contagious. Because you feel fine you are likely to be near people and talking to them in close proximity. This is where the virus shines. As you speak, microscopic spit balls filled with virions are floating through the air and those nearby are breathing them in.

Emergency rooms will normally see an spike in influenza this time of year in the US. A few days after friends and family congregated closely together for Thanksgiving.

Edit: Peak concentration is typically reached \*shortly after feeling symptoms\* rather than shortly before as stated above.