Aging is one of humanity’s most pressing challenges, yet we still lack a clear understanding of what it is or why it exists. This book presents the pathogen control theory of aging, which challenges conventional views. Rather than being a passive accumulation of damage, aging is proposed to be an evolved genetic program that removes older individuals, who are more likely to carry chronic infections and transmit them to kin. In this framework, aging functions as part of a lineage-level immune system–one that protects not the individual organism, but its relatives.