Some types of lung infections, known as lower respiratory tract infections, kill millions of people every year. Acute respiratory distress syndrome (ARDS) is the most severe form of lower respiratory tract infection. ARDS can result from a variety of conditions and is often life-threatening. The body's immune system responses kick-start the inflammatory process that contributes to ARDS. Existing treatments for ARDS are limited in their effectiveness. New drugs to treat ARDS are being developed and tested using knowledge about how the immune system works in the lungs. Taking steroids along with other treatments may be useful for some patients who have ARDS. Researchers have had significant success studying a protein called CD28 that mediates many aspects of immune system function. CD28-driven mechanisms have been shown to accelerate recovery from lung disease in mice and might improve outcomes for humans with similar conditions as well. Some bacteria take command of lung host cells and move into the airway surface liquid, a thin layer of liquid that coats the airways.