All Somerville fire companies and engines from Medford, Arlington and Boston responded to a reported fire at 31-33 Irvington Road Monday night at approximately 9:42 p.m. The two-unit condominium sustained severe damage and is currently deemed not habitable.  The blaze originated in a first floor bathroom ceiling and extended up to the attic inside the walls. A first floor resident was assisted by American Red Cross volunteers. The fire is not considered suspicious, possibly an electrical malfunction. However, at this time the cause remains undetermined. There were two minor injuries to firefighters.