Some diseases, like chicken pox, people only get once. why?

a) your skin keeps out the germs.
b) your allergens protect you.
c) the disease can only enter your body once.
d) some of the antibodies stay in your blood.

Some diseases, like chicken pox , people only get once because D.) some of the antibodies stay in your blood.. Therefore when or if you come into contact with the disease again it is highly unlikely for you to get it .

A is wrong because the skin of your body can spread the germs and considering that your mouth isnt covered by skin it would be impossible for your skin to keep everything out. And common sense says that if my skin keeps out germs then why would I have caught it in the first place. 

B is wrong because allergens irritate your body; you can go outside and be allergic to grass and still catch something regardless, if they were protecting your body then your body wouldnt try to reject them.

C is wrong but I dont feel like explaining (Someone else can)

And thus the correct answer is
D. Some of the antibodies stay in your blood