Live what Ivana is saying, this is a great question and something you don't think about quite often. They are not fully immune to viruses. They can easily get stolen, and bang - if your history is not cleared it can easily be retrieved. Be careful not to store personal info on these devices, as hard as this can be.
When you connect to the internet, which a tablet does, then there is a huge chance that you can also pick up a virus either through surfing the net or by email. Never thought about it really, but I think you can.
I believe that Apple does not have a problem with viruses, and as I have an i pad and not a generic tablet, I don't believe I am at risk getting a virus.
I also do not get emails on my i pad as I get them on my computer and phone so have no need to get them 3 times.
All my devices have protection. Mobile phones and tablets can install the free McAfee app just by going into Google store and searching for the free downloads. It doesn't seem to slow my tablet down and a small pop-up appears in the bottom left hand corner saying McAfee is protecting you and disappears every time you access the internet. I do find my mobile responds a little slower but better safe than sorry. I do also have a current paid subscription for my computer which covers me for everything.