spread, pervade, permeate, infiltrate

浸透(しんとう) Usually negative –  atheistic ideology is spreading in the youth. Communists are infiltrating all our shit. Can also be used in science, though:the solution permeates the membrane.     TTTSITU
 
蔓延(まんえん) Not so useful. Only used about the spread of diseases or harmful pests like kudzu.   SITU