Title: Understanding the Complexity of ATP Yield in Aerobic Respiration and the Impact of Microorganisms on Human Health

AT LEAST 100 WORDS PER QUESTION 
2 SEPERATE REFERENCES AFTER EACH QUESTION 
What is your reasoning behind the fact that the overall ATP yield in aerobic respiration is only a theoretical number?
Between viruses, fungi, and protozoans, which would you consider to be most harmful and most beneficial to mankind and why? Substantiate with examples for each.