Can a virus infect different computers connected to the same router?

[Emanuel N]

Brothers computer got hacked, he basically lost his yahoo/gmail/facebook accounts. His running a malware scan so that should be cleared up. However, whatever malware or virus that got his computer, can it infect other computers that are connected to the same router my brothers laptop uses? I mean ya, a malware program can come from anywhere, but can it spread through a router like that?