Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
An illustrative example of infection chains in our model. A virus transmits from a bird to a human (broken lines) then transmits from a human to another human and so on (solid lines). The xi represent the length of the ith transmission chain.
