Complex sentence: Usually this is localised (for example an infected spot on the scalp will cause lymph nodes in the neck on that same side to swell up), but when two or more lymph node groups are involved, it is called "generalized lymphadenopathy".
Split into simpler sentences: Usually this is localised (for example, an infected spot on the scalp will cause lymph nodes in the neck on that same side to swell). However, when two or more lymph node groups are involved, it is called "generalized lymphadenopathy".