Shared wifi signals make it easier for someone to hack into your phone, some even set your phone up as a shared point to help boost the signal within the area (Walmart's does not, it is fairly secure compared to some)

Think about your home network how you can see files on your computer on another one etc, it can make your phone like that giving access to passwords etc

Then again, if a hacker wants your info, they will get it

Virus warnings are less of an issue than the theft of data though, I can't think of anyone I personally know who got a serious virus on their phone, even from downloads

Basic info and tips on using public wifi here: http://blog.laptopmag.com/how-to-avo...-wi-fi-dangers