A couple years ago, the last time I was sick, was the only time I took the bus into work (excluding one other time).

Although my commute is only 8km, I couldn't muster the energy. We were also running bare-bones staff, so in actuality, you almost had to be on death's door to call in sick.

As a carfree individual, this is one of the downsides, as mentioned above. However, the upside is I'm rarely sick. And I believe that the minimum 40 minutes per day that I spend on the bike, to be a contributing factor.