i would imagine that most bike commuters here who are car-free (like me) still have a plan B to get to work in a pinch. i suppose there are some car-free folks who live out in rural areas that have no public transit what-so-ever, but most of us car-free people live in more urban/suburban/town environments where there's usually a bus route somewhere nearby at a minimum. yeah, it might take a while, especially if you have to do some wacky bus-transfer-bus-transfer-bus multi-stage transit commute, but it can usually be done.

for me, i ride my 30 mile round trip commute the vast majority of the time, but if i wake up and there's a thunderstorm going on outside or a foot of freshly fallen snow, i have a very easy train ride to work that only takes a little bit longer than biking. if i ride into work and the weather takes a turn for the worse in the afternoon, i can either try to wait out the storm, or just throw in the towel and ride the train home with my bike (the CTA lets bikes on trains after 6:30pm, so i just have to wait around at work for a little bit and it all works).

i would also like to add that now that i'm heading into year 4 of daily bike commuting, i rarely get sick anymore (knock on wood). some of that has to do with the fact that i'm taking better care of myself overall (eating right instead of the old pizza & beer diet of my 20s), but i have to believe that the 100 minutes of daily cardio that my body gets simply going to work and back has helped boost my overall health and in turn helped to keep my immune system firing on all cylinders.