In classical science fiction, alien attacks are perpetrated by oddly shaped creatures from outer space. Since the turn of the century, the genre has taken a more disturbing turn: invaders are earthly viruses. Although fictional, these new plots are based on a threat already experienced by a reliable majority: influenza, a well-known virus as small as 100 nm, infects millions of people every year, spreading the flu worldwide via airborne or direct transmission.

One of these fictional plots could feature Sarah, a young virologist facing the spread of an unknown virus threatening humanity. After a first observation of the symptoms and the evolution of patients, she sits at the microscope to focus on virus–cell interactions. Such interactions are fundamental to initiate an infection, since viruses need the machinery of the cell to be replicated and expressed.

Hers is a first-class standard microscope. It has the best components and offers images of the best possible resolution, but it is limited by the basic laws of light ray transmission. These are the same principles that explain rainbows and lenses, and were already beautifully summarized by Pierre Fermat in The Principle of Least Time: “Out of all the possible paths that light might take to go from one point to another, it takes the path that requires the shortest time.” Since Fermat, scientists have learnt that this principle is not completely general, but they have put forward modern refined versions that are.

When Sarah switches on the white lamp under the sample dish, the rays take Fermat's path, bending and refracting as they travel through different mediums in a labyrinth of crystal lenses, air, and mirrors. As a result, a greyish image of deformed cells reaches her eyes, indicating the infection is on course. She needs to understand how the virus is attacking; otherwise she will not be able to find a way to block or divert its maneuvers before every human dies. However, the image of the cells, an amalgam of indistinct moving grey shapes, does not give enough contrast to clearly identify the agents involved in the invasion.