History: How we caught cooties

A worldwide public health alert has been issued: for cooties.
Plague of many a playground, this highly contagious (not to mention icky) condition is transmissible via direct contact among the shrieking 6-11-year-old age group.
Anti-cootie measures include maintaining a safe distance from members of the opposite sex, as well as ensuring not to get caught by the aforementioned boys/girls during life-or-death games of tag.
Worse still, this serious scourge doesn’t seem to have any outward manifestations and is unlikely to show up on diagnostic exams.