  Because of its relevance to everyday life, the spreading of viral infections
has been of central interest in a variety of scientific communities involved in
fighting, preventing and theoretically interpreting epidemic processes. Recent
large scale observations have resulted in major discoveries concerning the
overall features of the spreading process in systems with highly mobile
susceptible units, but virtually no data are available about observations of
infection spreading for a very large number of immobile units. Here we present
the first detailed quantitative documentation of percolation-type viral
epidemics in a highly reproducible in vitro system consisting of tens of
thousands of virtually motionless cells. We use a confluent astroglial
monolayer in a Petri dish and induce productive infection in a limited number
of cells with a genetically modified herpesvirus strain. This approach allows
extreme high resolution tracking of the spatio-temporal development of the
epidemic. We show that a simple model is capable of reproducing the basic
features of our observations, i.e., the observed behaviour is likely to be
applicable to many different kinds of systems. Statistical physics inspired
approaches to our data, such as fractal dimension of the infected clusters as
well as their size distribution, seem to fit into a percolation theory based
interpretation. We suggest that our observations may be used to model epidemics
in more complex systems, which are difficult to study in isolation.
