Estimating fitness variation among microorganisms, meaning their aptitude to survive and reproduce in given conditions, allows researchers to predict their infection trajectories in single hosts and transmission in host populations. Among two viral strains, which will be the one to win against the host's immune response, or upon administration of drugs and vaccines? In virus dynamics, understanding such scenarios in detail is crucial, given the increase in resistance to antivirals and other evolutionary changes. Today, this understanding is enhanced via mathematical models, but the majority of current approaches describe limited scenarios, focusing on competitive exclusion, where one strain of the virus always wins over another because it has higher fitness.