Over that past 100 years the general pattern of mortality in the United States has gradual transitioned from acute infectious disease to chronic diseases. For example, deaths from childhood infectious such as diphtheria, pertussis, and scarlet fever (a Streptococcal infection) have declined dramatically. In addition to death from tuberculosis have dropped significantly. What is most to account for this transition?

answer; i believe the answer is moderate level;

                                                        water

a colorless, transparent, odorless, tasteless liquid that forms the seas, lakes, rivers, and rain and is the basis of the fluids of living organisms.

                                                        halon  

any of a number of unreactive gaseous compounds of carbon with bromine and other halogens, used in fire extinguishers, but now known to damage the ozone layer.

                                                  dry chemical

extinguishing agent used on class a, class b, and class c fires.

                                                  carbon dioxide

a colorless, odorless gas produced by burning carbon and organic compounds and by respiration. it is naturally present in air (about 0.03 percent) and is absorbed by plants in photosynthesis.