Do not have internal control of their body temperature
Body temperature is generally like the temperature of the environment
Have relatively low metabolic rates when resting, so their bodies do not generate much heat
Most reptiles, invertebrates, fishes, and amphibians are ectotherms that regulate body temperature primarily by absorbing heat from, or losing heat to, their environment
Compounds called pyrogen are produced and circulate to the hypothalamus resetting the thermostat to a higher value, allowing the body's temperature to increase to a new homeostatic equilibrium point in what is commonly called a fever
The increase in body heat makes the body less optimal for bacterial growth and increases the activities of cells so they are better able to fight the infection