When you’re sick with a fever, your doctor will likely tell you it’s a sign that your immune system is defending you against an infection. Fever typically results from immune cells at infected sites sending chemical signals to the brain to raise the set point of your body’s thermostat. So, you feel chills when the fever starts and feel hot when the fever breaks.
However, if you were to ask your doctor exactly how fever protects you, don’t expect a completely satisfactory answer.
Despite scientific consensus that fever is beneficial in fighting infections, exactly how is contentious. We are a veterinary pathologist and an emergency physician interested in applying evolutionary principles to medical problems. The evolution of fever is a classic conundrum because fever’s effects seem so harmful. Besides making you feel uncomfortable, you may also worry you’ll dangerously overheat. It is also metabolically costly to generate that much heat.
In our research and review, we propose that since fever occurs throughout much of the animal kingdom, this costly response must have benefits or it never would have evolved or been retained across species over time. We highlight several important but rarely considered points that help explain how the heat of fever helps your body fight infections.
Fever is a physiological response that has persisted for hundreds of millions of years across species.
How fever fights infection
Infections are caused by pathogens. Pathogens can be microbes such as certain species of bacteria, fungi or protozoans. If microbes or viruses have infected your cells and are using them to replicate, your own cells can also be considered pathogens and are treated that way by your immune system.
The main explanation for how fever helps control infections is that higher temperatures put heat-induced stress on pathogens, killing them or at least inhibiting their growth. But why would the somewhat higher body temperatures of fever – an increase of about 1.8 to 5.4 degrees Fahrenheit (1 to 4 degrees Celsius) – which can’t even kill your own healthy cells, harm such a wide variety of pathogens?
Immunologists have noted that slight heat makes immune cells work better. The implication is that fever is needed to enhance their defensive function. However, from an evolutionary perspective, it seems strange to require the massive energy cost of generating a fever just to get more activity from immune cells, especially since there are already plentiful and faster molecular signals available to activate them.
In addition to heat, slightly low oxygen levels and slight acidity also boost immune cell function. Since these stressful conditions also occur at infected sites, it makes sense that immune cells evolved to have their maximum functionality match their stressful working conditions. In fact, since anything in a state of growth is inherently…
