A fever is a high temperature. As a general rule, in children a temperature over 38C (100.4F) is a fever.
Reactions to hair dye are avoidable if you follow some simple safety advice. This page offers advice to anyone who uses permanent or semi-permanent hair dye, especially the darker-coloured dyes.
Hairy cell leukaemia is a rare type of chronic leukaemia that gets its name from the hair-like strands around the cancerous cells.
Read about hallucinations, including the possible causes and practical advice about what to do if you experience them.
Read about hamstring injuries, including the different types, how they're treated and how long it takes to recover.
Read about hand, foot and mouth disease. Find out what the symptoms are, how to treat it, and when you should get medical advice.
Read about the main types of cancer that affect the head and neck including mouth cancer, thyroid cancer and laryngeal cancer.
Health anxiety (hypochondria) is when worry about your health starts to take over your life. Includes advice about symptoms and treatments.
Read about when hearing tests are routinely offered to children, what they involve and why they're important.
Find out about heart-lung transplant, including when it is done, assessment and suitability, what happens during and after the operation, plus recovery and risks.
Read out about the main causes of heart palpitations and ectopic beats and find out when to get medical advice.
Heat exhaustion and heatstroke are two potentially serious conditions that can occur if you get too hot. They usually happen during a heatwave or in a hot climate, but can also occur when you're doing very strenuous physical exercise.
Read about prickly heat (miliaria rubra), an itchy rash of small, raised red spots that causes a stinging or prickling sensation on the skin.
Henoch-Schönlein purpura (HSP) is a rare condition in which blood vessels become inflamed. It typically results in a rash and joint and tummy pain.
Hepatitis is the term used to describe inflammation of the liver. It's usually the result of a viral infection or liver damage caused by drinking alcohol.
Read about herbal medicines, including what the possible risks are, who shouldn't use them, and what to look for when buying them.
Find out about Herceptin (trastuzumab), including how it works, when it's used, how it's given and what the possible side effects are.
Read information on hereditary haemorrhagic telangiectasia (Osler-Weber-Rendu syndrome), including symptoms, causes and how it's treated.
Hereditary neuropathy with pressure palsies (HNPP) is an inherited nerve disorder that typically causes numbness, tingling and muscle weakness.
Find out about hereditary spastic paraplegia (HSP), a rare group of inherited disorders that often cause weakness and stiffness in the leg muscles, which gradually gets worse over time.
Read about hernias, which occur when an internal part of the body pushes through a weakness in the muscle or surrounding tissue wall.
Find out about eye infections caused by the herpes simplex virus, including what the symptoms are, where to get help and what the treatments are.
Information on herpetic whitlow (whitlow finger), with links to other useful resources
Find out about haemophilus influenzae type b (Hib), a bacterium that can cause a number of serious illnesses, particularly in young children.
Find out about hidradenitis suppurativa, a painful, long-term skin condition that causes abscesses and scarring on the skin.
Read about hyperglycaemia (high blood sugar), a common problem that affects people with diabetes. Find out its causes, symptoms and treatments.
Read about hip pain in adults, including the possible causes, when to get medical advice, and how to manage it at home.
Hirschsprung's disease is a rare bowel condition that mainly affects babies and young children. Read about the symptoms, causes and treatments.
Read about hoarding disorders, including why it might happen, why it's a problem and what can be done about it.
Everything you need to know about homeopathy, including its uses and evidence base, with links to other resources.
Information about home oxygen therapy, also known as home oxygen treatment, including when it's needed, how it's given, the assessment process, and safety advice.
Homocystinuria (HCU) is a rare but potentially serious inherited condition where the body can't process the amino acid methionine.
Hypnotherapy is a type of complementary therapy that uses hypnosis, which is an altered state of consciousness.
Read about hypothermia, a dangerous drop in body temperature below 35C (normal body temperature is around 37C). Go to A&E or call 999 immediately if someone has the symptoms of hypothermia.
Irritable hip, also known as transient or toxic synovitis, is a common childhood condition that causes symptoms such as hip pain and limping.
Find out about haemochromatosis, an inherited condition where iron levels in the body slowly build up over many years.
Find out about the causes, symptoms and treatment of haemophilia, an inherited condition that means you bleed more than usual.
Alopecia is the general medical term for hair loss. There are many types of hair loss with different symptoms and causes.
Find out about when hand tendon repair is needed, how surgery is carried out, the recovery process and possible complications.
If you're considering having an operation or your GP has suggested you may need surgery, this guide is for you.
Read about the symptoms and causes of hay fever, and who's affected. Also discusses the various treatment options.
Find out how to spot head lice, how to get rid of them and how they spread.
Find out about the signs of hearing loss and deafness, what can cause it, when to get medical help and what the main treatments are.
Hearing tests are used to assess your ability to hear different sounds and to determine if there are any problems.
Find out about heart attacks, a serious medical emergency in which the supply of blood to the heart is suddenly blocked, usually by a blood clot
In people with heart block, the electrical pulses that control the heart rate are disrupted, causing the heart to beat more slowly.
Heartburn is a burning feeling in the chest caused by stomach acid travelling up towards the throat (acid reflux). If it keeps happening, it’s called gastro-oesophageal reflux disease (GORD).
Find out what heart failure is, including what the symptoms are and how it's treated.
Find out why heart transplants are carried out, what the procedure involves, what the risks are, and what you can expect while you recover.
Read about heavy periods, also known as menorrhagia. Heavy periods are where an excessive amount of blood is lost during several consecutive periods.
Hepatitis A is an infection that is rare in the UK. Find out how you get it, what the symptoms are, who's at risk, when to get medical advice, plus treatment and vaccination advice.
Read about hepatitis B, an infection of the liver caused by a virus. Find out about the symptoms, causes, treatments and risks of the condition.
Read about hepatitis C, a virus that can infect and damage the liver. Find out about the symptoms, causes and treatments for the condition.
A hiatus hernia is when part of your stomach moves up into your chest. It’s very common if you’re over 50. It doesn’t normally need treatment if it’s not causing you problems.
Read about things you can do yourself to stop or prevent hiccups.
Find everything you need to know about high blood pressure, including causes, symptoms, diagnosis and treatment, with links to other useful resources.
Find out what cholesterol is, what causes high cholesterol, why it's important to keep your cholesterol levels under control, and how to lower your cholesterol.
Read about hip fractures, also called proximal femoral fractures. Hip fractures are cracks or breaks in the top of the thigh bone (femur) close to the hip joint.
Read about hip replacements, a common type of surgery where a damaged hip joint is replaced with an artificial one (known as an implant).
Read about HIV and AIDS. HIV is a virus most commonly caught by having unprotected sex or sharing infected needles and other injecting equipment.
Urticaria – also known as hives, weals, welts or nettle rash – is a raised, itchy rash that appears on the skin. It may appear on one part of the body or be spread across large areas.
Hodgkin lymphoma is an uncommon cancer that develops in the lymphatic system, which is a network of vessels and glands spread throughout your body.
Find out about hormone replacement therapy (HRT) - how it can help menopausal symptoms, the different types of HRT, and what the main risks and side effects are - including how it can raise your risk of breast cancer.
Huntington's disease is an inherited condition that damages certain nerve cells in the brain.
Find out about hydrocephalus (fluid on the brain), including information about symptoms, causes, types, diagnosis, treatment and complications.
Read about hydronephrosis, a condition where one or both kidneys become stretched and swollen as a result of a build-up of urine inside them.
Find out what hypotonia is, what causes it, plus how it's diagnosed and treated.
A hysterectomy is a surgical procedure to remove the womb (uterus). You'll no longer be able to get pregnant after the operation.
A hysteroscopy is a procedure used to examine the inside of the womb. Find out why they're carried out, what happens during and after the procedure, and what the risks are.
Information and advice about head injuries and concussion, including the typical symptoms and what to look out for, plus how to treat minor head injuries at home.
Where to get help if you are having suicidal thoughts, and what to do if you are worried about someone else.
Read about some of the main types of headaches, and find out when you should seek medical advice.
Hypoparathyroidism is a rare condition where the parathyroid glands, which are in the neck near the thyroid gland, produce too little parathyroid hormone.
Many women experience headaches caused by changes in their hormones. Read self-help tips and treatments for hormone headaches, also called menstrual migraines.
Information on what HPV is, how it's spread, what conditions it can cause, and how to protect against it.
Choose which area of your hand hurts most to read about treatments for hand pain, when to get medical help and possible causes.
NHS information and advice about a high temperature (fever) in adults and children over 5, including what a fever is and how to tell if you have a high temperature.