This illustration captures the idea that our ability to handle environmental and other stressors is limited.
Exceeding the capacity of the system leads to symptoms and signs of illness.
The barrel teaches us that it is often combinations of stressors which lead to what we perceive of as a disease. For example, asthma can be associated with a combination of respiratory tract infection, low magnesium levels, dust mite and cockroach allergy, egg intolerance and sensitivity to chemicals.
Since frequently there is not just one thing putting load on the system, we can easily make the mistake thinking that the last thing added was the causative factor – there may however be many other things "in the barrel".
"Lowering the level in the barrel", so to speak, can be done by removing things from inside the barrel or by making the barrel bigger. For example, in the case of asthma mentioned above, treating the infection, adding daily magnesium supplements, putting dust covers on pillows and mattresses, desensitizing for dust mite and cockroach allergy, removing eggs from the diet and minimizing chemical exposures would be positive contributions that would either minimize the need for medications or greatly improve the effectiveness of medications.