They are fairly common, more folks notice them now that cameras are so prevalent. The growth is called a seroma, which is "bag" or pocket of skin filled with body fluid (mostly sera). Usually caused by an injury (fighting, fence, etc.). But, it is not infectious. Also not contagious, so no worries about contact with other deer. If they get too large, they can rupture and drain, which can sometimes cause an infection (like any other injury) and/or abscess.
The recreational value of game is inverse to the artificiality of its origin - Aldo Leopold