Yes, it's contagious, but the incubation period can be very long. DH and I commuted on our bikes in the Bay Area for years, mostly to the bafflement of his family. We left the country; now both his brothers are commuting by bike at least a couple of days a week, and both their spouses have recently acquired bikes.

The bug comes and goes, too. Until recently, I hadn't biked for months. Now I seem to be in some sort of acute phase of the affliction.