Is it listed as 2k,3k, or 4k class? Make sure it's in the biology section of your university or else it may not count. Specifically for the MCAT, I think reading comprehension from the psych class will benefit you more. However, if infectious disease touches on cell biology at the beginning and it's your weak point, you may consider taking it. There won't be straight up infectious disease passages on the MCAT, but you can bet there will be some sort of cell biology questions.