How does salivary gland infection originate, what are the symptoms and how is it treated?

Infection. These infections are rare. However, if your immune system goes down, then you could get wide range of viral and bacterial infections. The bacteria has to go against the flow of the saliva. So in cases where there is a stone in the duct, bacteria can easily build up and infect the gland.
Check this out. This is too long of a question to answer here. Go to the mayo clinic website. They have an excellent page on salivary gland infections.