What if transmission isn’t a single event, but an evolving ecological process?

What is it about?

In our PLOS Pathogens opinion article, we propose a framework that reimagines parasite transmission as a three-stage sequence, each shaped by distinct ecological pressures and evolutionary trade-offs: 1. Within-host infectiousness — how well parasites grow and prepare for release (e.g. spore production, replication rate, quality of the parasite offspring). 2. The environmental phase — survival and dispersal outside the host, influenced by abiotic factors like temperature or humidity, and biotic factors like vectors or competitors. 3. New-host infection — the capacity to initiate infection in a new host, shaped by spore quality and host susceptibility. This breakdown enables us to analyze the evolution of parasite fitness and virulence not only within hosts but also across their entire transmission pathway. For example, strong selection for environmental survival may constrain within-host replication, while increased within-host growth might reduce spore viability outside the host. These ecological trade-offs help explain how parasites adapt to different transmission regimes and environments.

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Photo by CDC on Unsplash

Photo by CDC on Unsplash

Why is it important?

From an evolutionary medicine perspective, this framework clarifies how interventions—like reducing environmental transmission or altering host availability—can shift selection pressures in unexpected ways. By identifying which transmission stage limits parasite fitness, we can better predict the direction of evolution and the likelihood of unintended consequences, such as increased virulence.

Perspectives