Interactions between viruses can occur when infecting the same host cell. These interactions can alter dynamical parameters of infection ultimately affecting the ecological and evolutionary dynamics of viruses and their hosts. In this thesis, I address through theory and experiment the causes and effects of multiple viral infections at the scale of the population. First, through individual-based models, I show that spatial clustering of host and viral populations emerge dynamically by varying key viral parameters related to the dispersal of progeny phage. Associated with the population clustering are increased numbers of hosts with multiple infections and increased numbers of viruses infecting the average host cell. Next, I address the effects of competition between viruses by modeling the population dynamics of a tripartite system, including host, virus, and virophage. I show the presence of virophage, a hyper-parasite of the virus, reduces the abundances of its competing virus while increasing the abundances of the mutual host. Lastly, I present and analyze experimental time series arising due to the infection of a host by a giant virus. By considering different ratios of inoculant viruses and hosts, I propose a novel method of viral growth with a strong dependence on the number of infecting viruses. Overall, I argue that multiple infections play a major role in shaping host and viral interactions in situ.
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