The immune system is a critical factor of host survival, allowing resistance to infections and maintaining tissue integrity. The activation of immune responses requires precise regulation to assure a balance between the benefits and costs of these responses. Moreover, the theory of antagonistic pleiotropy proposes that traits beneficial to early-life fitness may sustain costs that manifest later in life, after the period of strongest natural selection, where aging introduces further complexities for cooperative defenses. As a result of this, hosts of different ages may manifest distinct disease courses despite infection with the same pathogen.