One side of my husbands throat is swollen. what can this be?

Can be infectious: One can have one sided tonsillitis or pharyngitis. If this isn't associated with sore throat symptoms, you should see an ent asap. Also if there is any breathing issues, drooling, severe difficulty with opening your mouth i would see an ent or the er.l, as one can develop an abscess. Otherwise you can see your primary to see if antibiotics are warranted