The distinction between organic and inorganic carbon compounds, while useful in organizing the vast subject of chemistry... is somewhat arbitrary. Organic chemistry is the science concerned with all aspects of organic compounds. Organic synthesis is the methodology of their preparation.
No, it is not possible to contract the typical STDs, such as syphilis, gonorrhea, chlamydia, hepatitis B, etc. from wearing someone else's dirty underwear. However crabs and scabies, may be spread in this fashion.
When you wear underwear for the most part of 24 hours it can cause bacteria. The ideal way to sleep for a woman is to sleep with a bra on and underwear off. Gynecologists are saying that you should have support for your breasts for the biggest part of 24 hours.