The objective of this pilot study is to determine if ticagrelor therapy in individuals with pneumonia reduces markers of platelet activation, platelet-leukocyte aggregates, inflammation, acute lung injury, and lung mechanics. Because the benefit of anti-platelet therapy may the greatest in patients with more significant lung injury, the investigators will enroll patients with community-acquired pneumonia (CAP) requiring hospitalization or patients with hospital acquired pneumonia (HAP) within 48 hours of diagnosis. On study day 1, subjects will be randomized to receive ticagrelor (180 mg load and 90 mg BID) or placebo. Study medication (ticagrelor or placebo) will be administered twice daily on days 2 - 7 or until hospital discharge, if sooner than 7 days. Blood will be collected and assays performed on day 1 prior to study medication administration (baseline), day 2, 3, 7, day of discharge (if before 7 days), and 30 days for analysis of platelet count, markers of platelet activation, platelet - leukocyte interactions, biomarkers of inflammation, and measurements of lung mechanics.