  The renowned general epidemic process describes the stochastic evolution of a
population of individuals which are either susceptible, infected or dead. A
second order phase transition belonging to the universality class of dynamic
isotropic percolation lies between endemic or pandemic behavior of the process.
We generalize the general epidemic process by introducing a fourth kind of
individuals, viz. individuals which are weakened by the process but not yet
infected. This sensibilization gives rise to a mechanism that introduces a
global instability in the spreading of the process and therefore opens the
possibility of a discontinuous transition in addition to the usual continuous
percolation transition. The tricritical point separating the lines of first and
second order transitions constitutes a new universality class, namely the
universality class of tricritical dynamic isotropic percolation. Using
renormalized field theory we work out a detailed scaling description of this
universality class. We calculate the scaling exponents in an
$\epsilon$-expansion below the upper critical dimension $d_{c}=5$ for various
observables describing tricritical percolation clusters and their spreading
properties. In a remarkable contrast to the usual percolation transition, the
exponents $\beta$ and ${\beta}^{\prime}$ governing the two order parameters,
viz. the mean density and the percolation probability, turn out to be different
at the tricritical point. In addition to the scaling exponents we calculate for
all our static and dynamic observables logarithmic corrections to the
mean-field scaling behavior at $d_c=5$.
