Reassortment
If any of a virus’s genome segments is damaged in such a way as to prevent replication or expression of an essential gene, the virus is inviable when it, alone, infects a host cell (single infection).  However when two or more damaged viruses infect the same cell (multiple infection), the infection may succeed due to reassortment of segments. Each of the eight genome segments may be present in at least one undamaged copy.