For the sheep of St. Kilda, growing old brings with it a late-life decline in immune resistance against pervasive parasitic worms, which greatly reduces the animal's chances of surviving overwinter, regardless of overall physiological condition. The study's findings, which compiled the life histories of hundreds of wild sheep into a 26-year record of life and death, offer new insight into the little-understood role of immunosenescence in the wild. How and why we age remains a perennial mystery in life sciences, and much of what we know about immunosenescence - the steady deterioration of immune function with age - is largely derived from human populations and laboratory rodent models. While it's thought that this process also plays a role in shaping the morbidity and mortality of natural populations, particularly as it relates to parasite infections, the large-scale longitudinal datasets required to evaluate such questions are lacking. However, according to Hannah Froy and colleagues, the lifetime monitoring of the sheep on St. Kilda provided the unique opportunity to study the interactions of immune function, parasite burden, health and mortality. Froy et al compiled the fates of 800 individual animals, each facing natural aging and infection pressures. The authors discovered that individual aged sheep showed a decline in the availability of an antibody associated with protection against helminth parasite infection, which predicted mortality risk in the wild and their probability of surviving the winter. "[Froy et al.'s] results unambiguously demonstrate that long-term studies are not only essential to answer major ecological and evolutionary questions but also provide an invaluable resource to address topical scientific questions from the entire life science spectrum," writes Jean-Michel Gaillard and Jean-Francois Lemaitre in a related Perspective.