Fig. 1

Interactions between the environment and a biological system. The system can be any organism or group of inter-dependent organisms and the environmental exposure can be any changes of the environment. The image in a applies to the static part of susceptibility and in b applies to the variable part of susceptibility. (a) Basic relationship between exposures, effects in a biological system and the susceptibility factors characteristic of the system. Susceptibility is basically an effect modifier for how the exposure(s) affect the biological response. (b) The effect imposed upon the biological system may eventually change the system characteristics thereby changing its susceptibility. (c) The exposure of the system may also be directly affected by the system susceptibility factors themselves, e.g. if exposure is avoided or exacerbated (e.g. if the sensation of hunger is increased so food intake increases beyond needs)