Mumps is caused by the mumps virus, which belongs to a family of viruses known as paramyxoviruses. These viruses are a common source of infection, particularly in children.
When you get mumps, the virus moves from your respiratory tract (your nose, mouth and throat) into your parotid glands (saliva-producing glands found either side of your face), where it begins to reproduce. This causes the glands to swell.
The virus can also enter your cerebrospinal fluid (CSF), which is the fluid that surrounds and protects your brain and spine. Once the virus has entered the CSF, it can spread to other parts of your body, such as your brain, pancreas, testicles (in boys and men) and ovaries (in girls and women).