In order to infect a healthy cell, a flu virus must enter the cell (upper left) and empty viral genes into it. First, the virus binds to the cell. Then it becomes enveloped inside a bubble called an endosome as it gets taken into the cell. It is acidic inside the endosome--more acidic than the interior of the virus it contains. This pH difference serves as a signal to the virus that it is inside the cell, so it is time to release the viral genes (shown as orange squiggles). It is the M2 proton channel that helps the virus sense this difference in acidity and trigger the release of viral genes into the cell. Since the cell cannot tell the difference between its own genes and those of the virus, it gets tricked into making copies of the virus which can eventually go on to infect other cells.
Credit: Nicolle Rager Fuller, National Science Foundation