to be struck with a defiling skin disease: to break out in the scaly, spreading condition that made a person ceremonially unclean and forced them to live apart from others.
This word describes a person struck with a particular skin condition. It never appears as a plain action a person does; every time, it marks someone already afflicted, "a person with the disease." The related noun names the condition itself (צָרַעַת); this word is the state of carrying it.
The condition mattered less for its exact medical symptoms than for what it did to a person's standing. Someone struck with it was declared ceremonially unclean, had to tear their clothes and cover the lip and cry out a warning, and was sent to live outside the camp until a priest could pronounce them clean again. That is why the same word runs through both the ritual laws of Leviticus and the dramatic stories: Miriam, King Uzziah, Naaman the Aramean commander, Gehazi, and the four desperate men at the gate of Samaria are all named by it.
Across all these settings the word means the same thing. What changes is the story around it. Sometimes the affliction falls as a sudden judgment from God (Miriam, Uzziah, Gehazi); sometimes it is simply a condition a person lives with (the men at the gate); and once it is the thing a person is miraculously cleansed of (Naaman). The word itself only ever marks that someone is under this condition, and with it, cut off from ordinary life and worship.