Why 14 days, exactly?
The answer has to do with how viruses penetrate the cells and replicate them.
Once a virus infects someone a host it takes some time for the virus to make enough copies of itself that the host begins to shed the virus through coughs or sneezes (That's the way the host helps the virus spread to other people who are then new hosts.) This is the virus' incubation period.
For the hosts, it's generally the time between the first infection to the shedding of virus and the incubation period varies from virus to virus and sometimes from host to host says Rachel Graham, a virologist at the University of North Carolina's Gillings School of Global Public Health.