Annihilations are of course not decays, but they too occur via virtual particles.
In an annihilation a matter
and an antimatter particle
completely annihilate into energy.
That is, they interact with each
other, converting the energy of their
previous existence into a very energetic
force carrier particle (a gluon, W/Z, or photon).
These force carriers, in turn,
are transformed into other particles.
Quite often, physicists will annihilate two
particles at tremendous energies in
order to create new, massive particles.