Fever can be caused due to:
Infections, such as flu and pneumonia
Cancer
Teething (in babies)
Inflammatory diseases such as rheumatoid arthritis
Vaccines, such as those against tetanus or diphtheria (in children)
Certain medications, such as antibiotics or hypertension medicines
Blood clots
Heat stroke
Food poisoning
Hormonal disorders such as hyperthyroidism
Illicit use of drugs

Fever, by itself, is not a dangerous condition. An increase in body temperature usually indicates that the immune system is trying to fight and resolve an infection. However, in some cases, the temperature may rise too high, which can be serious and result in complications.

Childhood vaccinations such as those for diphtheria, or tetanus can lead to fever. Vaccines are products made from the causative organism of a disease, its products, or from a synthetic substitute. When a vaccine is given, it can cause an infection causing the immune system to get activated, and this may, sometimes, lead to fever.

To diagnose the exact cause of fever, your doctor will perform a physical exam and ask you questions about any symptoms or medical history. Based on it, your doctor will order certain diagnostic tests such as urine test, blood tests, X-rays, or other imaging tests.

The doctors usually prescribe an over-the-counter medicine, such as acetaminophen or nonsteroidal anti-inflammatory drugs such as ibuprofen. If your doctor suspects a bacterial infection as the cause of fever, then an antibiotic will be prescribed. In infants younger than 28 days old, fever may be due to a serious infection and they are usually given intravenous medicines.