There are two important medical reasons why it makes sense to use bicycles:
1. It is good for our health, because of its preventive effects against cardiovascular, pulmonary , metabolic, joint- and muscle- and bone diseases (including osteoporosis) and even diseases of the nervous sysem as well as the immune system. Even some kind of cancers seem to appear less frequently among cyclists, according to recent studies.
2. Using the bicycle instead of a car we can drastically reduce the production of green hous gases, a major factor of climate change. So we can reduce the health risks due to climate change which affect mainly people in developing countries.
Consider: According to UNDPs Human Development Report 2007/2008 the number of people affected by hydrometeorological catastrophies like storms, inundations and desertation has climbed from 38 millions per year in the early 70ies to an average of 262 millions per year in the period from 2000 to 2004.
Former UN Secretary General Kofi Annan's special commission on climate change in June 2009 presented a study according to which we have alrady got a loss of 300.000 persons per year due to climate change effects, mainly in DCs. And this at a level of 0,8 degrees increase of average temperature. We know that an increase of 2,0 degrees is unavoidable, not even 3,0 or 4,0 degrees if we don't change our life style dramatically and immediately.
For further information on economic questions of the necessary "Global Deal" read:
Nicholas Stern: A Blueprint For A Safer Planet. How to manage climate change and create a new era of progressand prosperity. London, The Bodley Head. 2009