Answer
Neuraminidase is an enzyme which is critical for the entry and release of viral particles into host cells, so Tamiflu would either prevent any entry into host cells of an uninfected person or prevent new viral particles from lysed cells from infecting more cells in the same host. The first case prevents someone from having the flu, and the second case shortens the duration of infection by limiting reinfection.
Work Step by Step
The key here is to see that this drug affects viral entry into cells, which limits overall infection if taken before infection or during the early stages of the flu.
