The old wives' tale "feed a cold, starve a fever" really does work, according to a new scientific study.
Sick people lose appetite, suffer lethargy, fever, sleep disruption and depression, stop washing and retreat to bed.
Researchers at Yale University in the United States tested the old maxim on mice with a cold caused by a virus or fevers caused by bacteria.
They found why anorexia helps the organism to tolerate bacterial infections but makes viral infections hard to endure.
Experiments found mice with bacterial infections died when fed while those with viral infections who were fed lived.
They found, particularly, sugar - but not fat or protein - led to the death of mice infected with listeria, a bug that causes food poisoning
Professor Ruslan Medzhitov said: "We were surprised at how profound the effects of feeding were, both positive and negative
"Anorexia - not eating - is a common behaviour during sickness that is seen in people and all kinds of animals.
"Our findings show that it has a strong protective effect with certain infections, but not with others."
In the first series of experiments, mice were infected with the bacterium Listeria monocytogenes, which commonly causes food poisoning.
The mice stopped eating, and they eventually recovered.
But when the mice were force fed, they died.
The researchers then broke the food down by component and found fatal reactions when the mice were given glucose, but not when they were fed proteins or fats.
Giving mice the chemical 2-DG, which prevents glucose metabolism, was enough to rescue even mice who were fed glucose and allowed them to survive the infection.
When the experiment was repeated in mice with flu, they found the opposite effect.
Those infected with the flu virus A/WSN/33 survived when they were force fed glucose, but died when they were denied food or given 2-DG.
It was found different areas of the brain were affected depending on which type of infection the mice died from, indicating that the animals' metabolic needs differ depending on which part of the immune system had been activated.
Prof Medzhitov added: "Almost everything we know about infection is based on immune response studies and looking at how the immune system eliminates pathogens.
"But that's not the only way we defend ourselves.
"There are also cases where we change and adapt so that microbes don't cause harm.
"Our study manipulated the ability of these mice to tolerate and survive infection without doing anything that had an effect on the pathogens themselves."
Further research is planned into the effects of disrupted sleep, another common sickness behaviour affects how the immune system fights infection.
It is also looking into why people crave certain foods when they're sick.
The findings have important implications for people suffering from blood poisoning which is life threatening as the body's immune system goes into overdrive as it tries to fight an infection.
This can reduce the blood supply to vital organs such as the brain, heart and kidneys and without quick treatment, sepsis can lead to multiple organ failure and death.
The study was published in the journal Cell.