The methods described herein are based on the observation that oscillations in breath isotope ratio data can be used for the purpose of identifying an "unhealthy" state in an organism such as a human. Described herein are methods of determining the state of health of an individual, such as the transition from healthy to infected, by identifying changes in oscillation modes in breath isotope ratio data. Changes in the frequency and/or amplitude of the oscillation modes are correlated with the heath of the individual. The methods can advantageously be used to provide information about the health of an individual in shorter periods of time than previous methods.