An untreated AIDS patient (CD4+ count of 180 cells/mm3) from southern California has developed progressively severe headache and mental confusion, along with ataxia and retinochoroiditis. Focal lesions are present on a computed tomography scan of his brain. No mucocutaneous lesions are found. He has been living under a bridge for the past 2 years. His level of immunoglobulin G (IgG) to the infectious agent is high. What is the most likely explanation for how this current infection staed?