Between care and contagion: Insights from the endotoxin model into the social facets of sickness

The ability to recognize sick and potentially contagious conspecifics is crucial for survival, particularly in social species where close contact increases the risk for disease transmission. This creates an evolutionary trade-off between avoiding infection and maintaining care for sick group members. This narrative review summarizes research using bacterial endotoxin (lipopolysaccharide, LPS) to experimentally induce sickness, focusing on its effects on social behavior in animals and humans. LPS-treated animals generally show reduced social exploration of healthy conspecifics, while healthy conspecifics tend to avoid them. Such avoidance behavior is influenced by environmental factors such as housing conditions, health status, and social hierarchy. Some species, when sick, show a preference for familiar individuals, and exhibit more affiliative, less aggressive behaviors. In humans, LPS-induced sickness leads to heightened sensitivity to both positive and negative social cues, which may reflect an adaptive response to increased vulnerability. Individuals under LPS also demonstrate an enhanced ability to regulate emotional responses and reduced empathy for others' psychological pain, suggesting a shift towards a more self-focused, energy-conserving state. Sick individuals additionally tend to seek care from those with a history of supportive behavior. Humans can detect sickness in others through olfactory and visual cues, such as odor, facial expressions and posture. As observed during the COVID-19 pandemic, prolonged social isolation negatively affects both infected individuals and their caregivers. Future research should therefore investigate the impact of sickness on higher level social cognitive functioning, as well as the role of modulating variables such as familiarity, sickness severity and sample demographics.