Why We Feel ‘Sick’: MIT’s Zuri Sullivan Decodes the Brain-Immune Connection Behind Infection Symptoms

Why do we lose our appetite and feel tired when sick? MIT's Zuri Sullivan explains how the brain orchestrates sickness as an adaptive immune defense.

By: AXL Media

Published: Feb 26, 2026, 8:47 AM EST

Source: The information in this article was sourced from MIT News

The Biology of Being 'Ill'

Everyone recognizes the symptoms: a sudden lack of appetite, the urge to cancel plans, and a deep, crushing fatigue. While we often view these as the unfortunate side effects of a virus, Zuri Sullivan sees them as a vital part of our immune defense. Sullivan, who recently joined the MIT Department of Biology and the Whitehead Institute, works at the multidisciplinary intersection of neuroscience, immunology, and physiology. Her research posits that sickness is "immunity at the organismal scale"—a system-wide shift in state that the brain initiates after receiving signals from the immune system.

Sickness as an Adaptive Strategy

The core of Sullivan’s hypothesis is that sickness behaviors are evolved, adaptive processes. For example, a fever isn't just a sign of inflammation; it's a controlled physiological response that makes the body less hospitable to pathogens. Similarly, loss of appetite—an area Sullivan focused on during her doctoral work at Yale—might be the body's way of depriving a pathogen of the nutrients it needs to replicate. By understanding how the immune system "talks" to the brain’s hypothalamus (the region that regulates hunger, sleep, and temperature), Sullivan hopes to uncover exactly how these behaviors contribute to host defense.

The Mystery of Social Withdrawal

One of the most fascinating aspects of sickness is the behavioral shift toward social isolation. "Too sick to socialize" is a common phrase, but the biological triggers are complex. Sullivan is collaborating with other researchers, including Sebastian Lourido, to examine how specific pathogens like Toxoplasma gondii can influence social behavior. By studying a wide range of invaders—viruses, bacteria, and parasites—her lab aims to determine if different sickness "profiles" are tailored to fight specific types of infections.