More than anyone wants to know:
Bubonic plague can get into the bloodstream (septicemic plague) and infect the lungs, causing pneumonia (pneumonic plague) which can then be transmitted person-to-person by "respiratory droplets" (coughing). Bubonic plague is transmitted by fleas feeding off infected rats (also any rodents such as ground squirrels, bunnies and cats) and then biting people. Overcrowding in cities contributes greatly to its spread, which is why moving to the country might have been a reasonable idea.

The incubation period is 1-7 days, but if a person with pneumonic plague isn't coughing, it doesn't seem too easy for them to transmit the disease, but I can see how it could be possible. The incubation period is the time between being infected and beginning to have symptoms - not when the person becomes infectious. It's possible they're infectious before they begin feeling sick. The book I have, Beneson's Control of Communicable Diseases Manual, doesn't say anything about this. I think pneumonic plague is so rare they may not have had a chance to study it much. There were only 12 cases of plague in the US from 1984-1983, and no cases of person-to-person transmission since 1925.