Why Do I Feel Horny When I Have a Cold?

Don’t laugh, but I get extremely randy whenever I have a cold. Am I the only one? Why does this happen? – Mike

Dear Mike,

It seems counter-intuitive to be lookin’ for love when your body is under attack, but you are not alone. When I searched Google for phrases such as “sick and horny” I found men and women wondering why they feel frisky when they’re fighting infection.

How prevalent is it? Darned if I know. What causes it? I don’t know that either. It turns out that there is very little research on immuno-induced friskiness.

But surprisingly, I stumbled across an obscure study from 1995 that touches on your question and suggests that there may be biochemical mechanism for what you experience. Let’s come back to that. First we need to establish some background on your sexy immune system.

Sex and the Modern Immune System

Did you know that men are at higher risk than women for major infections after surgery? Or that women are at higher risk for autoimmune problems? (Offner et al., 1999; Ahmed et al., 1985.)

These immune system differences are related to testosterone – the rocket fuel of reproduction. Other things being equal, more testosterone means less immunity. (Interestingly, the researchers pointed out that men’s increased risk of infection can be reversed by castration. So can your average guy’s reason for living.)

Across many species, males pay a price for the possession of testosterone. (Women have testosterone too, but in lower levels.) The male red grouse, for example, suffers more parasite infections during the spring when his testosterone is elevated (Mougeot et al., 2005). Other bird species suffer similar testosterone-related vulnerability.

There are different theories for the lackluster immune performance of otherwise virile birds. Some researchers believe their bodies must make sacrifices when allocating resources, and so they pay the price of reduced immunity in order to produce secondary sex characteristics like ornamental feathers that attract females. Others believe that testosterone actively suppresses immune function so that only the most viable animals survive long enough to reproduce (Munoz et al., 2008; Roberts et al., 2009).

Whatever the cause, cocks pay a price for virility. (A cock is a male bird. What did you think I meant?)

Human males pay a similar price. Testosterone and the effectiveness of one’s immune system are linked, and women appear to be the beneficiaries – sort of. Pre-menopausal women, with their low testosterone levels, have a stronger adaptive immunity to the common cold than men (Carroll et al., 2010). Unfortunately, there are no free lunches in life. More sensitive immune systems make women more prone to the aforementioned autoimmune problems.

Clearly, sex hormones and the immune system are intertwined. Sex can even boost your immunity because orgasm increases the number of leukocytes in the blood (Haake et al., 2004). It’s all a rich tapestry, to quote a minor character from The Simpsons.

You may have noticed that I am straying from your question, Mike. You didn’t ask me to explain how sex hormones affect immune response, you asked how the immune response affects sex drive.

Welcome to my world. That is precisely the frustration I encountered when researching your question. There is plenty of research on the effects of sex hormones, but very little concerning your question. We’re going to have to dig a little deeper.

Immunity May Be the Sixth Sense

In a saucy editorial (saucy for a professional journal, anyway) Felice Bedford (2011) argued that the immune system should be counted as one of our senses, along with taste, vision, and hearing. She was drawing on a body of research that describes how the immune system detects foreign bodies, sends signals to the brain, and drives behavior.

She makes a compelling case. If a baseball player’s visual system detects a bean-ball headed his way, the player’s behavior will change. He will get out of the way rather than swinging the bat.

In a not dissimilar fashion, a person’s immune system affects behavior when it detects injury or infection (Maier & Watkins, 2000). Rather than communicating via nerve bundles, the immune system uses cytokines, which are messenger molecules similar to hormones, to communicate with the brain.

Researchers Larson & Dunn (2001) discovered that cytokines produce several behavioral changes. Theirs was a very sensuous paper with erotic phrases like this:

“…cytokines may act on brain endothelial cells activating cyclooxygenase, enabling the synthesis of prostaglandins, which could readily cross the endothelial barrier to penetrate the brain parenchyma.”

Ooh, baby! Talk dirty to me.

Their findings suggest that cytokines released during an immune response can bypass normal channels and directly affect the brain. According to Larson & Dunn, cytokines cause the brain to reduce food intake, physical activity, and social behavior. They change sleep patterns, impair certain cognitive functions, and suppress pleasure-driven behavior. They turn us into dopey, unmotivated grumps.

Your question is particularly interesting, Mike, because an increase in sex drive contradicts what we know about the the immune response. An activated immune system typically depresses behavior rather than revving our engines.

But there appears to be an exception. In a dark corner of an arcane 1995 journal article is an inscrutable phrase that may hold the germ of a beginning of a slightly possible answer to your question. After suffering through at least 20 articles on the subject – none of which approached your question – I was plenty intrigued to find this little gem:

“Post hoc tests indicated that male [rats] injected with the high dose of IL-1β had significantly higher PP score than those injected with saline” (Yirmiya et al. 1995).

I was in nerd heaven. Here’s a translation. Recall from your high school biology class that macrophages are immune system cells that cruise around the body, cleaning up messes and attacking troublemakers.

When macrophages become activated, as they do when attacking an invader, they release a cytokine (the aforementioned messenger molecule) called interleukin-1. Interleukin-1 suppresses sexual behaviors in female rats, but has no such effect on males. In fact, high doses of interleukin-1 increased male rats’ motivation to find a willing mate. (Other sexual behaviors remained mostly unaffected.)

You might wonder why interleukin-1 has this unlikely effect on sexual behavior. Is it an evolutionary adaptation that compels male rats to mate one last time before they die? A cunning scheme by which nefarious viruses branch out to other warm bodies? A mere quirk of the rat’s immune system?

Those questions bring us back to square one. I don’t know why the immune system would have that effect. I don’t even know if this explains what you experience when you’re sick, Mike. It certainly doesn’t explain why women in my Google search described a similar experience, why my friends and family denied the experience, or why several of them slapped me for bringing it up.

But it is an indication that the immune response, by way of some complex mechanism, has the potential to be as stimulating as Charlize Theron or David Hasselhoff, depending on your groove.

All I can say for certain is that the immune system is bafflingly complex (to me, at least) and has a permeating influence over our behavior. I’ve done my best, and now I turn the matter over to any adventurous endocrinologists who wish to weigh in, because I give up.

-IS

PS – I’m curious to hear from others who have this experience since the research is so scant. Does infection make you randy, baby? Do share. But keep it clean!

References

Ahmed, S.A., Penhale, W.J., & Talal, N. (1985). Sex hormones, immune response, and autoimmune disease. American Journal of Pathology, 121, 531-551.

Bedford, F.L. (2011). The missing sense modality: the immune system. Perception, 40, 1265-1267.

Carroll, M.L., Yerkovich, S.T., Pritchard, A.L., Davies, J.M., and Upham, J.W. (2010). Adaptive immunity to rhinoviruses: sex and age matter. Respiratory Research, 11, 184-193.

Haake, P., Krueger, T.H., Goebel, M.U., Heberling, K.M., Hartmann, U., and Schedlowski, M. (2004). Effects of sexual arousal on lymphocyte subset circulation and cytokine production in man. Neuroimmunomodulation, 11(5), 293-298.

Larson, S.J., & Dunn, A.J. (2001). Behavioral effects of cytokines. Brain, Behavior, and Immunity, 15, 371-387.

Maier, S.F., & Watkins, L.R. (2000). The immune system as a sensory system: implications for psychology. Current Directions in Psychological Research, 9, 98-102.

Mougeot, F., Redpath, S.M., & Piertney, S.B. (2005). Elevated spring testosterone increases parasite intensity in male red grouse. Behavioral Ecology, 17, 117-125.

Munoz, A., Aparicio, J.M., Bonal, R. (2008). Male barn swallows use different resource allocation rules to produce ornamental tail feathers. Behavioral Ecology, 19(2), 404-409.

Offner, P.J., Moore, E.E., & Biffle, W.L. (1999). Male gender is a risk factor for major infections after surgery. Archives of Surgery, 134, 935-949.

Roberts, M.L., Buchanan, K.L., Evans, M.R., Marin, R.H., and Satterlee, D.G. (2009). The effects of testosterone on immune function in quail selected for divergent plasma corticosterone response. The Journal of Experimental Biology, 212, 3125-3131.

Yirmiya. R., Avitsur, R, Donchin, O, and Cohen, E. (1995). Interleukin-1 inhibits sexual behavior in female but not in male rats. Brain, Behavior, and Immunity, 9, 220-233.