Real Dictionary

invasive
any impairment of normal physiological function affecting all or part of an organism, esp a specific pathological change caused by infection, stress, etc, producing characteristic symptoms; illness or sickness in general [...]
latent but unrealized ability or capacity [...]
of or relating to an invasion, intrusion, etc [...]
You use invasive to describe something undesirable which spreads very quickly and which is very difficult to stop from spreading.