That weary feeling that sets in with an illness is an emotion that helps you fight off infection, says a University of Oregon research team.

Slack facial muscles and drooping eyelids appear early. Exhaustion, loss of appetite and increased sensitivity to cold and pain come on. Those signs are among a long list of features that can be tied to the emotion of being sick, which the authors label lassitude, a now little-used term for weariness taken from Latin in the 16th century.

In the journal Evolution and Human Behavior, three researchers in the UO Department of Anthropology argued that the state of being sick qualifies as an emotion following a review of the literature on sickness behavior, most of which has focused on behavioral and physiological changes in nonhuman animals.

In their paper, published online ahead of print this month, they merged the accrued knowledge from 130 published studies and proposed that lassitude is a complex adaptation, like the immune system, that evolved to help people fight infectious disease.

"The immune system clearly helps us fight off infections, but activating the immune system costs a lot of energy,” said UO doctoral student Joshua Schrock, the paper’s lead author. “This cost creates a series of predicaments for the body’s regulatory systems.”

Schrock’s co-authors, Josh Snodgrass and Lawrence Sugiyama, both professors in the Department of Anthropology, have addressed health issues in various field projects around the world, including an ongoing collaboration with the Shuar, an indigenous population in southeast Ecuador.

"Lassitude is the program that adjusts your body’s regulatory systems to set them up for fighting infection,” Schrock said. “These adjustments make you feel sadder, more fatigued, more easily nauseated, less hungry and more sensitive to cold and pain."

Lassitude, the UO team wrote, persists until the immune response subsides. During that response, various mechanisms are called upon to coordinate the fight against infection, which, they noted, can trigger symptoms resembling psychological depression.

During the battle, lassitude coordinates adjustments to patterns of movement, risk avoidance, body temperature, appetite and, even, how a person elicits caregiving behavior from social networks.

Lassitude, the researchers wrote, “modifies the cost-benefit structure of a wide range of decisions.” Those who are ill place lower value on food and sex, for example, and often prefer to avoid social and physical risks.

“When threat levels are high, the system sends a signal to various motivational systems, configuring them in ways that facilitate effective immunity and pathogen clearance,” they write in their conclusion. “We believe that investigating the information-processing structure of lassitude will contribute to a more complete understanding of sickness behavior, much like the information-processing structure of hunger helps us understand feeding behavior.”

While their paper focused primarily on illnesses triggered by bacteria, viruses, parasitic worms and protozoans, they also theorized that other situations – such as injuries, poisoning and chronic degenerative diseases – may present similar adaptive problems.

—By Jim Barlow, University Communications