The evolution and maintenance of cooperation is a fundamental problem in evolutionary biology. Because cooperative behaviors impose a cost, cooperators are vulnerable to exploitation by defectors that do not pay the cost to cooperate but still benefit from the cooperation of others. Viruses exhibit cooperative and defective phenotypes in infection: during replication, viruses produce essential gene products in the host cell environment. Co-infection between multiple viruses is possible, so if a virus cannot guarantee exclusive access to its own gene products, the products act as a public good; cooperators contribute to the common pool while defectors contribute less and instead appropriate goods from cooperators. Defective virus phenotypes experience negative frequency dependence in co-infection. For some viruses, negative frequency dependence is strong enough to maintain the cooperative phenotype. For other viruses, negative frequency dependence alone may not sufficient to maintain the cooperative phenotype, and the fate of cooperation is unclear. Here we propose that if co-infection is not enforced, and host and viral densities can vary, environmental feedback can maintain cooperation in such viral populations by modulating the rate of co-infection and shifting the advantages of cooperation versus defection. We build and analyze an ODE model and find that for a wide range of parameter values, environmental feedback maintains cooperation.