Scientists have discovered a possible evolutionary explanation for the placebo effect with new evidence suggesting the immune system has an on-off switch to save energy.

People who suffer from a weak infection often recover whether they take a medicinal drug or a simple sugar pill - which suggests humans can heal themselves. But this has begged the question why people need to wait for the placebo before the recovery process from an infection begins.

According to the New Scientist, researchers have now found that something similar to the placebo effect occurs in animals, after studying Siberian hamsters. If lights above the hamsters laboratory cages mimicked winter, they found the hamsters would not fight the infection. However, if the lighting was changed to replicate summer conditions, the hamsters mounted a full immune response.

Similar to this, people who think they are taking medicine to treat an illness, but are actually receiving a placebo, can see a response from their immune system twice that than people who take no pills. The evidence shows that intervention causes a mental response which kicks the immune system into action.

According to Peter Trimmer, a biologist at the University of Bristol, there is an explanation for this.

He suggests that the immune system uses up lots of energy when it is in action. So an animal's energy reserves cold be severely depleted if the immune system launches a long response to an illness. 

If the infection is not likely to causes death, it could be better to wait and see that fighting the illness will not put the animal in other dangers. Evidence from a computer model designed by Mr Trimmer and his colleagues now supports this evidence.

It found those animals which live in more challenging environments were food was harder to find, they lived longer if they put up with infections rather than launch a response from their immune system. However, for those animals living in much more favourable conditions, it was better for them to launch a response from their immune systems so they return to health quicker. 

This is because in better conditions they have more access to food which provides energy to sustain an immune response.