Fever

What are the symptoms of fever?

Medically, a fever is defined as a body temperature of 37.5°C or higher. However, because body temperature varies from person to person, a temperature below 37.4°C can also be considered a fever. In most cases, fever is caused by infection or inflammation, but there may also be complex underlying causes such as collagen disease, cancer, or endocrine abnormalities. Sometimes fever occurs as a normal response when the body is infected with bacteria or a virus and fights the microorganisms that cause the infection, so it is not necessarily bad for the body, and the idea that “all you need to do is lower the fever” is beginning to be reconsidered.

最后更新：2026年3月。由CLINIC TEN SHIBUYA医疗团队监修。

Things to keep in mind when you have a fever

Even if you visit a hospital immediately after the onset of a fever (such as within a day), the cause of the fever is often unknown. If you have a fever but no other symptoms such as loss of consciousness or difficulty breathing, you should consider using over-the-counter antipyretics or general cold medicines, and visiting a general outpatient clinic during the day from the day after symptoms appeared.

Also, since it is easy to become dehydrated when you have a fever, try to drink small amounts of fluids containing electrolytes and carbohydrates, such as sports drinks. If you have symptoms such as drowsiness or difficulty breathing in addition to a fever, you should immediately seek medical attention.

Possible causes of fever

Infectious disease

Most fevers are caused by bacterial or viral infections. If you were feeling fine for a few hours to a few days and then suddenly developed a fever, it is likely an infectious disease. Even if you have an infectious disease, it is most likely a cold or flu, but if you have any of the following symptoms, there is a possibility that another illness is lurking.

• Insect bites or bite marks
• Eating the following foods in the days before the fever started:
  • Raw meat, fish, and spoiled food
• Traveling overseas (especially to countries with poor sanitary conditions, such as Southeast Asia and Africa)
• Sexual intercourse (especially with a new partner)

However, even if you have no idea what the cause may be, it may be an infectious disease, so it is important to tell your doctor as much detail as possible about what happened.

Other diseases

While an infectious disease is often the trigger, if there is no particular cause in mind, the fever may be caused by one of the following illnesses:

• Collagen disease
  • Rheumatoid arthritis
  • Systemic lupus erythematosus
• Cancer and malignant tumors
• Endocrine disorders (hormonal abnormalities)
• Drug side effects
• Functional hyperthermia (a condition in which the body temperature rises due to stress or other psychological factors, even though there is no problem with the organs)

If you have a fever like this, you should see a doctor as it may require specialized treatment.

It is difficult to determine the cause of a fever from symptoms alone, but in any case, as mentioned above, we recommend that you wait for about a day and then consult a medical institution. Please be aware that some medical institutions are currently not able to treat fevers.

High-risk illnesses accompanied by fever

Among the illnesses that cause fever, there are some that are highly urgent and require immediate medical attention, such as acute epiglottitis, meningitis, peritonitis, septic shock, and severe pneumonia.

• Unconscious
• Difficulty breathing
• Severe pain regardless of location
• Within a few hours I was unable to stand up
• Unbearable chills and shivering

If you experience any of the above symptoms, please visit a hospital as soon as possible, including at night or at the emergency room.

summary

• Medically, fever is defined as a body temperature of 37.5°C or higher.
• Infectious diseases are the main cause, but some cases are caused by very serious illnesses.

常见问题

在日本发烧了怎么办？

如果体温超过38°C，请前往附近的诊所就诊。在日本，退烧药（如对乙酰氨基酚）可在药店无需处方购买。如果体温超过39°C或持续48小时以上，请务必就医。

在日本能买到退烧药吗？

可以。在日本的药妆店（Drug Store）可以无需处方购买含对乙酰氨基酚或布洛芬成分的退烧药。向药剂师说”退烧药（解热剂/げねつざい）”即可获得帮助。

在日本看发烧要花多少钱？

如果没有旅行保险，初诊费用大约为3,000～10,000日元。如需血液检查或其他检查，费用可能更高。如果您有旅行保险，大部分费用可以报销。

什么时候应该去急诊？

如果出现40°C以上的高烧、意识模糊、严重呼吸困难、剧烈头痛伴颈部僵硬、或伴有皮疹，请立即前往急诊。日本的急救电话号码是119。