When a flea bites it is actually sucking blood from your pet. A lot like dracula. When a flea infestation occurs the amount of blood loss to a pet can overwhelm their body's ability to produce enough Red Blood Cells (RBC's). RBC's are needed to carry oxygen throughout the body. RBC's are the primary vehicle that gets oxygen to organs.

When the RBC count gets low enough it is called anemia. That means the RBC count is less than 35-40%. It also means the body is severely oxygen deprived. Your pet will show signs of fatigue, inappetance, & depression because they do not have the energy to move.

When there is a lack of oxygen (hypoxia) for a long enough period of time cells will start to die. Should enough cells die within a certain organ you will see failure of that organ such as cardiac arrest in the heart or respiratory failure with the lungs.