HMN 2025: How Individual behavior matters more than we think in spreading epidemics

Even a small proportion of citizens who do not follow health measures can amplify the spread of an epidemic and make it expand faster in large cities.

These are the results of research conducted by the Politecnico di Milano and published in the journal Proceedings of the Royal Society A.

The study, authored by Fabio Mazza, Marco Brambilla, Carlo Piccardi and Francesco Pierri from the Department of Electronics, Information and Bioengineering (DEIB), analyzes how individuals who do not comply with public health recommendations influence the spread of contagions in urban environments.

Using contact networks based on real data and a mathematical model that distinguishes between citizens adhering and not adhering to public health measures, the researchers simulated the spread of an epidemic in three Italian cities: Turin, Milan and Palermo.

The simulations showed that even a small proportion of non-adherents can substantially increase the infection peak and hasten the time of infection, straining health care facilities. The effect is particularly evident when the transmissibility of the disease is moderate.

Moreover, when people who ignore preventive measures are concentrated in certain areas, local hotspots can form, making it more difficult to contain the epidemic.

“We observed that although the epidemic followed similar overall dynamics in the cities we studied, the geographical distribution of non-adherent behavior significantly alters local contagion trajectories, leading to marked differences between city districts,” explains Pierri, a researcher in the Data Science Lab and coordinator of the study.

This work emphasizes the need to monitor the distribution of preventive behavior and adapt public health strategies to the nature of individual urban contexts, thereby making public health interventions more effective.

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