Your Smart Phone is just like any other computer device. It can receive a virus from webpages you visit and any "iffy" e-mails your friends send you. Any 3rd party apps from unknown websites you may download. Or even that cute Kitty picture your Mom sends you in a text. (if you don't know where She got it, or her phone was already infested when she took the pic how can you tell it does not have a virus?
The ONLY way to tell if you have a virus is to have an anti-virus program running on your phone and the best and cheapest is a program called "Lookout". Cheapest because its FREE!!! they have a paid version that adds a few bells and whistles but the FREE version will protect your phone.
It will also help you locate your phone should it become lost or stolen. You install the program on your phone, and at that time sign up on their website. (<--For FREE) If your phone is lost you can log onto their website under your ID and password then select "FIND MY PHONE" and it will give you a Google map of yours phones location down to the accuracy of your phones GPS which many times is +/- 5 Feet.
You can also ring your phone from that site if you lost it around the house and don't have another phone to call it.
For the paid version you can also remotely ERASE everything on your phone. This would come in handy to prevent someone from accessing all your e-mails and private information. Or photos maybe you should not have. lol You can also brick the phone. that means whoever took it cannot use it at all. or make it squeal whenever its turned on..
I like this added (paid) feature knowing it would piss the thief off not being able to use the phone for anything.
As for telling if your phone has a virus. The ONLY way to know is to have an anti-virus tell you. Sure your phone may run slower than normal, or your friends may be getting e-mails or texts from your phone telling them to go to a website without you actually sending them. BUT MOST of the time you will notice NOTHING and the virus will quietly be collecting your credit card numbers and bank information and be sending it to the bad guys without your knowledge!
The modern Smart Phone contains more computer power that the desk tops of just 10-Years ago.
My advice: NEVER operate a computer of any kind without the proper protection software.
This also applies to the iPhone.
Apple does not advertise it because they want to give the impression that there "stuff" does not get viruses but they Make AND Sell anti virus software for their macs and phones. As more people bought macs it became popular enough for the bad guys to make money off spying on Macs. There are now many viruses written for the Macs, iPhone and iPad's. AND Apples sells the anti-Malware programs to protect their equipment. BUT believe it or not even some of the Apple iStore "GENIUSES" don't know about it. LOL!!! (and I use the term "Genius VERY loosely)