Field Notes Issue 1

Issue #1

When Systems Adapt

On flexibility, vectors, and persistence

Welcome to the first issue of Field Notes.

Each week, I take one idea from the episode—something that feels like a hinge point—and follow it where it leads. Not to repeat the science, but to see what it reveals.

In the Margins

What makes murine typhus unusual isn’t the pathogen—it’s the flexibility of the system. The transmission cycle used to be a closed loop of rats, fleas, and humans. Now the system includes opossums, pets, and suburban environments. Not because the pathogen changed—but because the vector didn’t need it to. 

Cat fleas move easily between species. They don’t specialize. And that one feature—flexibility—means the system doesn’t break when the environment changes. It adapts.

Not all infectious diseases can do this. Some are tightly constrained by their vectors. Chagas disease, for example, still depends on triatomine bugs. Without them, transmission stops—even if potential hosts are everywhere.

And murine typhus may not be the only system where this is happening. Any disease moving through cat fleas—and the animals that carry them—has the same opportunity to reorganize.

Underlined

For flea-borne systems like this, the early signals won’t look like outbreaks—not at first. They’ll look like shifts in the system:

• Fleas appearing more consistently in domestic animals (especially cats and dogs)

• Evidence of cat fleas carrying multiple pathogens

• Increased overlap between wildlife (like opossums), pets, and human spaces

• Human cases without clear or traditional exposure routes

Those quiet shifts matter more than a single spike in case counts.

What It Points To

The diseases that persist aren’t always the most aggressive—they’re the ones whose systems can adapt.

Postscript

Thank you for subscribing. This week in April always brings me back to why I do this work in the first place. It’s the tension between what we know—and how easily that knowledge can be missed, misapplied, or overlooked. 

Lately I’ve been thinking about that in a different context too—what happens when access to HIV medications is interrupted, and how quickly that reshapes risk for other infections like tuberculosis. Different system, similar pattern—the consequences of disruption don’t stay contained.

— Heather