Does my iPhone have a virus??

my iPhone is acting wonky...should I be worried about a virus? It's deleting random text message, not delivery others, will shut off for no reason...Would resetting it help or is it just because the phone is 3-4 years old & is past its prime?

I'm also wondering if this is from the last update I did to the 8.2...I was told not to update as it would screw up my phone since the newest updates are meant to work with the newest phones. Is there a way to fix this? I don't want to have to spend the money on a new phone yet...especially not the iPhone 6's they're HUGE!! I'm hoping for a better option in the iPhone7