A friend of mine (who I won't identify out of a desire to continue breathing) reports that illness often makes her randy.
Now, I can't recall that ever happening to me. All I've ever wanted when really sick was just to lie in bed and watch Star Wars, or read a book. However, other people have reported simular phenomenon.
What might be the mechanism? We know that certain parasites can change their hosts behavior in order enhance the parasite's reproduction. For example, the parasite Toxoplasmosis gondii can only complete its reproductive cycle in cats. But it can live in a variety of intermediate hosts in the meantime, including rats and humans. And it appears that T. gondii changes the behavior of both.
Normal rats avoid areas marked by cat urine. But rats infected with the parasite T.gondii are indifferent to cat urine. Some even seek it out. Obviously this increases the likelihood that the rat will be consumed by a cat, allowing T. gondii to reproduce.
In humans, high concentrations of antibodies to T. gondii have been correlated with schizophrenia and other neuorological disorders. It's still unclear to what extent T. gondii contributes to these disorders, or how they would enhance T. gondii's survival. But isn't it curious that one of the sterotypes of the mad is their fondness for cats? The "crazy cat lady" in the house down the street may be feeding her pride of 27 cats at the behest of a parasite infecting her brain.
And if T. gondii can influence behavior, why not viruses? After all, what better way to transmit a virus than to make your host want to swap bodily fluids with someone else?
And imagine if, through genetic engineering, the mechanism of action could be identified. Or enhanced. And made sex selective.
Imagine hordes of randy sick women ripping their clothes off and throwing themselves at the nearest shocked, but somewhat flattered male.
What a horrible day that would be for us all.