Prions are misfolded proteins that are infectious and naturally transmitted, causing a fatal neurological disease in humans and animals. Prion shedding routes have been shown to be modified by inflammation in excretory organs such as kidney. Here we show that sheep with scrapie and lentiviral mastitis secrete prions into the milk and infect nearly 90% of suckling naive lambs. Thus lentiviruses may enhance prion transmission, conceivably sustaining prion infections in flocks for generations. This study also indicates a risk of prion spread to sheep and potentially to other animals through dietary exposure to pooled sheep milk or milk products.