During the time of evolution of humans, the food supply was intermittent and for survival there was need to store energy in excess of what is required for immediate use. Fat cells did the job of storing energy, which are widely distributed in adipose tissues. The fat cells can store excess energy efficiently as triglyceride and, when needed, release stored energy as free fatty acids for use at other sites. This physiologic system developed during the time of evolution acts through endocrine and neural pathways and even can permit humans to survive starvation for as long as several months. But at present era of nutritional abundance, sedentary lifestyle, and influenced importantly by genetic endowment, this physiologic system of storing energy in adipose tissues can produce obesity and adverse health consequences due to obesity.

Blood and blood products transfusion can be life saving, but if not done appropriately and with appropriate precaution, blood transfusion can also cause problem to the recipient of the transfused blood. The complications that can occur due to blood transfusion includes, transmission of infectious agent (from donor to recipient), immunologically mediated adverse reactions (such as hemolysis, graft-versus-host reaction, anaphylactic reaction etc.), non-immunologically mediated adverse reactions such as hypothermia, fluid overload, electrolyte excess etc.

There are several infectious agents that can be transmitted through blood transfusion, but practically very rarely seen in clinical practice due to rigorous screening of blood for infectious agents before transfusion,