<prompt> Von Magnus phenomenon -Is a normal replicative cycleVirus yield has low hemagglutinationVirus has high infectivityVirus yields has high hemagglutination titre but low infectivity  <response>: Abnormal replicative cycles of virusesVon Magnus phenomenonwhen cells are infected with a high dose of influenza virus, there is defective assembly during replication and the produced virus has high hemagglutinin titre but low infectivity.Abortive infectionDue to defective maturation or assembly, either no release of virions occur or the progeny of a virus is noninfectious.Defective virusesSome viruses are genetically defective and require the help of simultaneously infected helper virus for replication. eg. HDV, Rous sarcoma virus. 