This chapter discusses nicotinamide adenine dinucleotide phosphate (NADPH) Oxidases and Reactive Oxygen in Viral Infections, with Emphasis on Influenza. Reactive oxygen species (ROS) in biological systems were originally thought to be produced mainly by accidental mechanisms such as by-products of metabolism, or from ingestion of toxins or chemicals that can induce redox cycling. In addition to phagocytes, ROS are produced by a variety of cell types and tissues, often in response to hormones and growth factors including insulin, angiotensin II, epidermal growth factor, and platelet-derived growth factor. Increased ROS can result from either increased production of radical species (for example by NADPH oxidases (NOX) enzymes or mitochondria) or a decrease in their removal by antioxidant systems. NOX enzyme expression is complex and dependent upon factors such as species, tissue, cell type, and physiological or pathological status. The chapter explains the Effects on ROS by HIV and HCV Proteins. Oxidative stress is also associated with infections by hepatitis C virus (HCV).