A virus is capable of infecting a closed system though an attached device -- a clever AI could install itself on various peripherals and gain access that way. Containment is a strong word, if AI virus is evasive it will penetrate any containment, just like biological viruses do. Also, devices are being increasingly more and more connected to each other, even if they are not per-se connected to the internet. All it takes is one super spreader... (maybe, this is just speculation)