"Researchers estimate that the non-human microorganisms in and on a human body outnumber the human cells by a factor of 10. More facts about non-human cells : Although more than 3 million types of microbes are thought to exist, fewer than 8,000 have been described".

Many of the cooties that can get you are already inside you. Your immune system can rise and fall depending on what it's subjected to and that allows the cooties to do you harm.