Every winter, these unwelcome symptoms — fever, aches and runny noses — make their appearance.

The influenza virus, which floats in the air, is the culprit behind all this. It hitches a ride on airborne droplets, entering the human body and launching a carefully orchestrated "invasion" of human cells.

In the past, scientists were unable to observe this process in detail because the viruses are too minute and too fast, and most microscopic techniques require the destruction of cells in order to magnify the image.

However, a team of scientists led by the Swiss Federal Institute of Technology Zurich has now used high-resolution microscopy to capture the entire process of the influenza virus entering living cells for the first time.

They were surprised to discover that this invasion is not simply "breaking and entering," but rather a complex and dynamic "dance" between the cell and the virus. Rather than being passive victims, the cells even "actively cooperate" with the virus's entry.

This may sound counterintuitive. However, it turns out that the cells "cooperate" because the cunning influenza virus "hijacks" a crucial everyday transport system within the cell. This system is the cell's lifeline and is responsible for transporting essential substances, such as hormones, cholesterol and iron, from outside the cell to inside it.

The influenza viruses then travel along the cell surface until they find an area with a high density of receptors — their ideal entrance.

When the receptors detect the virus attachment, a precise process of cellular engineering begins. The cell membrane at the point of attachment then starts to invaginate, forming a small pit. Clathrin proteins then arrive and deepen the pit, completely enclosing the virus in a vesicle. The cell's transport system then carries this vesicle, containing the "dangerous package," to the interior. Finally, the vesicle membrane dissolves and the virus is officially established inside the cell.

Surprisingly, cells also actively participate in this process. Not only do they rapidly mobilize clathrin proteins to the site of the virus to "construct" a structure, but their cell membranes also actively deform, undulating like waves in an attempt to "retain" viruses that are trying to leave, ensuring they are captured and brought inside.

These images had never been seen before. Thanks to a technology that combines atomic force microscopy and fluorescence microscopy, scientists can now finally track the detailed dynamic process of viruses entering cells.

This technology is not only applicable to influenza viruses, but is also expected to help people understand the invasion tactics of other viruses and observe how vaccines "train" immune cells in advance to prepare them for battle.