Viruses with a second envelope spike called fusion (F) glycoprotein as well as an attachment spike, e.g. canine distemper virus, make the plasma membranes of adjacent cells fuse to each other and to the lipid bilayer of the virus. These cells can eventually become multinucleate syncytia with about 30 nucleii before the replicating virus kills them. Fusion is also a means of virus spreading between cells. Antibody to fusion protein determines the success of vaccination.