  A simple, but ``classical``, stochastic model for epidemic spread in a
finite, but large, population is studied. The progress of the epidemic can be
divided into three different phases that requires different tools to analyse.
Initially the process is approximated by a branching process. It is discussed
for how long time this approximation is valid. When a non-negligible proportion
of the population is already infected the process can be studied using
differential equations. In a final phase the spread will fade out. The results
are used to investigate what happens if two strains of infectious agents, with
different potential for spread, are simultaneously introduced in a totally
susceptible population. It is assumed that an infection causes immunity, and
that a person can only be infected by one strain. The two epidemics will
initially develop approximately as independent branching processes. However, if
both strains causes large epidemics they will, due to immunity, eventually
interact. We will mainly be interested in the final outcome of the spread,
i.e., how large proportion of the population is infected by the different
strains.
