Very neat concept, well presented and executed.
Introducing one mechanic at a time works well to tutorialize without overwhelming the player with too much too soon. It does feel a bit easy, once all three abilities are unlocked, as the infected areas can quickly be nullified and cut off, long before the firewall countdown is complete.
It's also unclear if the shielded node can be attacked or not; I don't know if it's always an invincible bulwark or the means of losing the game if it's compromised. I played through assuming it was vulnerable and that protecting it was the win condition; however on the final level I realized I'd left it open to attack from one infected node, yet it wasn't targeted.