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In general younger people tend to have higher body temperatures, while older people run cooler. That's for a variety of reasons but for one, older people are often less active than younger, energetic folks. The energy used to run a mile or play a soccer match can increase our core body temperatures. That's also why we sweat so much during intense exercise, so that we can cool our bodies down. Our internal systems also run hotter during the day than they do at night.
By medical standards, a person has a fever if their temperature rises above 38.2 degrees Celsius (100.4 degrees Fahrenheit), although some people may have medically significant fevers below that temperature (as is often the case with elderly people, or very young infants). However, fever or it’s more official-sounding medical name “hyperthermia” is not a universally defined term. Some doctors refer to fevers as only elevated temperatures caused by infections and inflammation, while others use it to describe all situations where a person’s temperature rises above 100.4 degrees Fahrenheit.
The majority of fevers are caused by an infection. This can include not only viruses and bacterial infections, but also parasites, urinary tract infections, and appendicitis. Less common causes of infection, but causes all the same, include things like malignant tumors, tissue ischaemia (a condition in which not enough blood gets to a particular tissue in need), and reactions to certain drugs. Drugs that can cause reactions include illegal substances like Ecstasy, prescription drugs like antidepressants such as SSRIs, and in rare cases, antipsychotic medications. Other causes of fever include inflammation after surgery or from a brain injury, endocrine disorders, and even cancer.
