Most serious problems due to bacteria that invade while defences are weak. Death may occur in infants, elderly. Then RNA replicates in nucleus. Exit by budding out. Viruses shed for long time.
Antigenic shift occurs every few years.
Thought to result from mixing of different virus strains in same cell, recombination of surface protein genes to give new arrangements. Vaccines not good for more than a few years because of new strains. Enough DNA to code for about 70 proteins. DNA synthesis occurs in nucleus. Proteins synthesized in cytoplasm, travel back to nucleus for assembly.