Perturbation theory, Intermediate states, Virtual particles

The following is taken from page 13 of Peskin and Schroeder.

Any relativistic process cannot be assumed to be explained in terms of a single particle, since ##E=mc^{2}## allows for the creation of particle-antiparticle pairs. Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order perturbation theory. We can think of such states as existing only for a very short time, according to ##\Delta E \cdot{\Delta t}=\hbar##. As one goes to higher orders in perturbation theory, arbitrarily many such virtual particles can be created.

I have the following questions regarding the above paragraph:
  1. What is meant by a multiparticle state?
  2. What is meant by an intermediate state in second-order perturbation theory?
  3. Why do multiparticle states appear as intermediate states in second-order perturbation theory
  4. In what other ways do multiparticle states appear?
  5. What is the range of energies over which multiparticle states appear?
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A. Neumaier

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