If I give my cold to my brother why can’t he give it back to me?

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Why do I become immune to it?
How does contagiousness work anyways?

In: Biology

Because your body makes memory cells that remember the cold virus you had.. so the next time you are exposed to it your body is very good at quickly eliminating the virus before it can get you sick.

In general when you survive any disease, you are then immune from it.

Colds and flus evolve just enough that each year it’s a different enough disease and your immunity from the previous year doesn’t count. But in the short time from you to your brother and back again – it’s still the same cold you have learned immunity to, so you can’t get it again 🙂