"Please I want to know if cisco IOS/router configuration files can be infected by virus. "

Almost impossible. However, whether a router's config files might be corrupted by accessing the Cisco router from an infected host, where a virus has "captured" telnet, is a much more likely possibility.

A computer virus is targeted against certain host systems. Just as you don't normally see a virus that targets both PCs and MACs, one that "spreads" between Cisco routers and PCs would be unlikely.

However, if the virus in question has "captured" telnet, but still lets it operate, using an infected PC to access a Cisco router might indeed damage a config yet not "infect" the router.

If you've ID'ed the virus, you might want to research its behavior.