Does wearing a condom decrease the chance of getting an std? if so by how much?

Some/not all: If you look at the mechanics involved, the condom just reduces transfer of fluids and contact with the covered shaft of the penis. STD germs can cling to other surfaces involved in sex, and transfer to uncovered surfaces. Condoms reduce but do not eliminate your risk of STD's.The amount varies with sexual practices/germs involved/type of exposure so no firm number can be cited.