The invention relates to the diagnosis of disease or the determination of functioning of cellular organisms, being of multi-cellular or unicellular nature, being visible by the naked eye or being a microorganism. The invention provides a method for determining functioning of a cellular organism comprising determining the relative ratio of a first endosymbiont cellular organelle nucleic acid and/or gene product thereof in a sample obtained from the organism in relation to the amount of a second nucleic acid and/or gene product thereof.