Viruses spread between cells, tissues, and organisms by cell-free and cell-cell transmissions. Both mechanisms enhance disease development, but it is difficult to distinguish between them. Here, we analyzed the transmission mode of human adenovirus (HAdV) in monolayers of epithelial cells by wet laboratory experimentation and a computer simulation. Using live-cell fluorescence microscopy and replication-competent HAdV2 expressing
green fluorescent protein, we found that the spread of
infection invariably occurred after cell lysis. It was affected by convection and blocked by
neutralizing antibodies but was independent of second-round
infections. If cells were overlaid with
agarose, convection was blocked and round plaques developed around lytic infected cells. Infected cells that did not lyse did not give rise to plaques, highlighting the importance of cell-free transmission. Key parameters for cell-free virus transmission were the time from
infection to lysis, the dose of free viruses determining
infection probability, and the diffusion of single HAdV particles in aqueous medium. With these parameters, we developed an in silico model using multiscale hybrid dynamics, cellular automata, and particle strength exchange. This so-called white box model is based on experimentally determined parameters and reproduces
viral infection spreading as a function of the local concentration of free viruses. These analyses imply that the extent of lytic
infections can be determined by either direct plaque assays or can be predicted by calculations of virus diffusion constants and modeling.