For the first time, researchers have captured the moment a flu virus breaches a human cell — and what they found challenges how we think about infection itself.
A team from ETH Zurich and Japanese institutions developed a new imaging technique that lets them watch viral invasion happen in real time, at the molecular level. The discovery: your cells aren't passive victims. They actively reach toward the invading virus, pulling it inside through the same doorways they use to transport cholesterol and hormones.
"The infection of our body cells is like a dance between virus and cell," says Yohei Yamauchi, the molecular medicine professor who led the work. That metaphor matters. It reframes infection not as a simple breaking-and-entering, but as a hijacking of machinery your cells already use.
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Start Your News DetoxHow the virus finds its way in
Influenza viruses don't just slip through the cell membrane anywhere. They surf along the surface, bouncing from spot to spot until they find a cluster of receptor molecules — the entry points cells naturally offer to beneficial visitors. Once a virus latches on, the cell's membrane begins to dimple inward, cupping around the invader. A protein called clathrin reinforces this pocket, which deepens until it completely envelops the virus and pulls it inside.
Previous imaging methods couldn't show this process in motion. Electron microscopy destroys cells in the process, leaving only still frames. Fluorescence microscopy blurs the fine details. The new technique — called ViViD-AFM — combines atomic force microscopy with fluorescence imaging to reveal the choreography as it happens.
The researchers discovered something striking: when the virus drifts even slightly away from the cell surface, the membrane actively lifts upward to recapture it. Cells also position clathrin proteins exactly where the virus needs them. It's not that cells want to be infected. They're simply following their normal protocols, and the virus is expert at exploiting them.
Why this matters for medicine
This real-time view opens a new window for antiviral research. Instead of testing drugs on dead cells or in abstract models, scientists can now watch how potential treatments interfere with viral entry as it actually happens. The same technique could reveal how other viruses — or vaccines — interact with cells from the first moment of contact.
The work was published in September 2025 in the Proceedings of the National Academy of Sciences. What started as a way to understand flu infection better may become a standard tool for studying how pathogens breach our defenses.
