Yoga Flu and more

I recall one time hearing about something like a yoga flu. Do you know what I am talking about? What asanas bring it on. I had a student tell me that she felt sick in class; light headed and sick to her stomach; that it lasted for several days. I don't think we did anything strenuous enough to make anyone sick but told her I would check in to it.
Also in another class, I would call it a backbends class I ended with "the rack". One of the students thought she would like to do the rack first and then she would be better able to do the backbends.
Yours thoughts??

i have actuallly heard of two variations on the yoga flu. the first one strikes newcomers in mysore studying with the Jois family all the time. apparently when you push your body to the limit, particularly with a fair amount of heat, you create a little mini-fever that burns up bacteria in your system - which is good, right? well yes, but the fever happens very quickly and your body ends up with WAY too much dead bacteria to process. they begin to decompose and voila! you feel like absolute shite for a couple days. they call it mysore fever. pattabhi jois used to laugh about it and call it a "good cleansing".
the other, i suspect more common, yoga flu is a response to backbending. it is actually more of a really nasty headache than a flu. it hits you right behind the eyes. throbbing. awful. migraine-esque. it is made much worse when people strain their necks backwards (ustrasana, salabhasana come to mind) and try to lead the backbend with the neck instead of with the much more poweful muscles of the mid-back. I have heard a couple explanations. One is pressure on the optic nerve during backbends. The other is compression of the kidneys causing a "backwash" of toxins into the bloodstream. Neither one has ever been demonstrated scientifically. Another possibility is an extreme adrenal response to backbending, causing a sudden surge in cranial blood pressure (would be especially bad in an inverted backbend such as urdhva dhanurasana).
do any of you regularly get (or used to get) backbending headaches? every experienced a "good cleansing"? have you found any thing that helps to alleviate the backbend headache?

Booze.

No, just kidding! Well, mostly kidding. I can remember one of the worst headaches I ever had from yoga was after doing a long spell of rope sirsasana. I had been doing some backbending in the pose too and when I came out of it, I had the most intense pressure right between my eyes. It eventually dissipated to a dull throbbing by the end of class, but it was initially quite painfull. Normally, however, I find most regular backbending headaches are short lived. Usually I'm so focused on the subsequent poses that the headache doesn't really bother me. For the really bad ones though, I like to do balasana in a dark, quiet room.