An undissolved solute, I believe.
Gas exchange in humans is the delivery of oxygen lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs.
Soil, water and food are all nonliving reservoirs of infection
by the law of natural selection, daddy charles darwin stated that overpopulation could result in famine and wars, and death would accelerates across nations, so overpopulation is a cause for this because of too many organisms but not enough food, which declined growth rate.