Virus

A virus is a small infectious agent that replicates only inside the living cells of other organisms.

Effects on cellular respiration
Some viral infections may affect mitochondrial function and reduce cellular respiration. A study of poliovirus found that polio infection rapidly decreases (45% decrease) cellular oxygen consumption (and thus energy production through cellular respiration) by inhibiting succinate dehydrogenase (55% decrease in activity) and blocking mitochondrial electron transport at or between complexes I and III.