I quiz them about "what kind of sick", and if they say v, then the panic ensues!!
If I cannot get away from the situation (for example if I am stuck sitting beside them in a lecture or something), then I just sit there with my heart pounding, getting hot and cold flashes, sometimes I feel like I will pass out, and my thoughts are racing.
Once I can finally get myself out of the situation, I panic for the next few days! And the next time I see the friend, I ask them if they feel better (and pray that they say yes and that they never got sick in the first place!!)