1. To invade in great numbers, as of pests; SYN. overrun.
2. To live on or in a host, as of parasites.
ETYM Latin obsessus, p. p. of obsidere to besiege; ob (see Ob-) + sedere to sit.
To preoccupy completely; to take all one's attention.
To engage in obsessive thinking; become obsessed with an idea.