I think it has something to do with the animation but i can't figure out how you did the animation. I can't do a check if the animation is playing or not. Sorry still learning myself.
Personally i would just make a sprite with 2 frames. A normal one and a infected one. that way you can check which frame is set.
Though i am interested how you did the animation because i don't see where its coming from. <img src="smileys/smiley1.gif" border="0" align="middle" />