Same happened to me few months ago, when I started using snapchat on my Samsung Galaxy S5. The phone would heat up to the max at first, few weeks later the phone would shutdown by itself when the battery was anywhere bellow 35%. Then it started happening when using other apps as well, but I'm sure Snapchat was still running on the background, that's what would make the phone switch off when using other apps. I also thought it was a virus. Almost considered doing a reset on my phone. But I ended up leaving it like that for a couple more weeks and, my battery eventually swollen because of the heat. So I got a new battery. But here I am almost two months with the new battery and I'm facing the same problem. Snapchat has to do something about that, I can't keep on changing batteries every 2 months. I saw that closing background apps including snapchat when I'm not using helps ease up the heat and resistance of the battery. Hope we all find a solution for this!!! take care