poison ivy

How on earth does a 1 1/2 year old get poison ivy?  I think we are never to know.  LM has a raging case from the bottom of his shorts down to his little chubby ankles.  He has been great about it, not scratching at all.  He patiently lifts his legs for lotion several times  a day and sometime even points to the spots and say boo boo.  He also loves the extra time in the oatmeal bath playing with his toys.  I myself get horrible poison ivy that spreads like crazy!  I am thrilled that with his first case, he has managed to not look like a freak (like mama) with it all over his body.  The mystery is where did he get it?  No one at daycare or any of the neighborhood kids have it.  Big Daddy and I have been looking for it in the yard, but have not found any yet.  I fear we are never to know where it came from, just that our Little Man is a trooper!