Mumps is a disease caused by the mumps virus, which is a member of the paramyxoviridae family.
This is actually a large family of viruses which includes measles virus and parainfluenza viruses, and all of these tend to affect children the most.
Mumps only affects humans and is spread by tiny respiratory droplets that are small enough to be carried short distances in the air, so mumps virus is extremely contagious and anyone near a person with mumps is at risk for getting the disease as well.
The mumps virus has a single strand of RNA and a viral polymerase enzyme surrounded by a phospholipid bilayer envelope studded with viral proteins hemagglutinin-neuraminidase, or HN protein, and fusion or F protein.
The HN protein allows the virus to stick to a potential host cell, and cut itself loose if necessary, and the F-protein which fuses the viral and cell membranes together allowing the mumps virus to enter the cell.
Once mumps enters a cell, the single stranded RNA, which is negative sense, gets transcribed by the viral polymerase enzyme, into a complementary positive sense strand of mRNA, which can then be translated by the host cell ribosomes into new copies of the envelope proteins and the viral polymerase, which get assembled into new viruses.
What also ends up happening with these, though, is that those HN and F-proteins on the cell surface now bind other cells, so they actually end up bind epithelial cells to one another, which forms a clump of connected cells called a multinucleated giant cell or a syncytium.
Mumps enters the body and first infects the epithelial cells of the nasopharynx, where it starts replicating and causing local damage to the tissue.
From there, it can cause viremia or virus in the blood, and reach various organs and tissues.
The mumps virus has tropism, or preference for, the parotid salivary glands, and the most classic finding in mumps is swelling of parotid salivary glands either on one side or on both sides, sometimes with an associated earache.