If you've popped your blister, the best way to know if your blister is infected is from the colour of the fluid. It should always be clear. "If the fluid is white or yellow, the blister may be infected and needs medical attention," says Dr. Frieling. If the blister has not been popped, it may be hard to distinguish if it's infected or not, but there are signs that can still help determine if that's the case, including warmth in the area, a foul smell, pus, or swelling. You should also see a professional immediately if the blister doesn't seem to be healing on its own because even a small infection can spread to other areas of the body. Cellulitis, which is a potentially serious skin infection that spreads rapidly, can also develop. You'll know its cellulitis if you see a red streak moving up your leg, and you should seek emergency attention.