Infectious diseases: what they are, how they spread, symptoms and when to get help

An infectious disease is an illness caused by a germ that gets into the body and multiplies. The germs most people have heard of are viruses and bacteria, and fungi cause a further group of infections. NHS inform's infections hub lists everything from the common cold, flu, chickenpox and norovirus to tuberculosis, meningitis and urinary tract infections, which gives a sense of how wide the range is.

Some infections are mild and short-lived. Others are serious or life-threatening, and some can be passed on easily, which is why public health bodies track them. This page is a general guide. For a specific infection, the condition guides linked below go into more detail.

Symptoms depend on where the infection is and what is causing it. A high temperature is a common general sign. The NHS says a high temperature is usually 38C or above, that you may feel hot, cold or shivery, sweat, or have warm, red skin, and that you can have a fever even if a thermometer reads below 38C if you feel hot, cold or shivery. It also says a fever is often just the body fighting an infection such as a cold or flu, but it could be a sign of something more serious if it is very high or will not come down.

Other symptoms follow the part of the body involved. For example, NHS inform describes sores and crusts for impetigo, and the NHS lists coughing, choking and a wet-sounding voice for swallowing problems. The sepsis symptoms below apply to any serious infection.

• Fever, chills or shivering.

• Cough, sore throat or breathlessness (respiratory infections).

• Pain or stinging when passing urine (urinary infections).

• Red, hot or crusted skin (skin infections).

• Vomiting or diarrhoea (stomach and gut infections).

• A rash that does not fade when you press it, which can be an emergency sign (see below).

Infections spread in several ways. NHS pages describe skin-to-skin contact and touching infected items such as towels and bedding for impetigo, and the NHS says washing hands removes dirt, viruses and bacteria to stop them spreading to other people and objects, including illnesses such as food poisoning, flu or diarrhoea. Other infections are linked to food, travel or mosquitoes; the NHS pregnancy page describes yellow fever as a virus spread by mosquitoes.

Whether you become unwell depends on the germ and on you. The NHS lists things that raise the chance of sepsis after an infection: being under 12 months or over 75, being pregnant or having recently given birth, having a weakened immune system (for example because of HIV, sickle cell disease, chemotherapy, corticosteroids or immunosuppressants), having a wound, burn or skin infection, having a catheter, or injecting drugs or misusing alcohol.

For most infections, rest, fluids and paracetamol or ibuprofen if you feel uncomfortable are enough, and the NHS says to try to stay at home and avoid contact with other people until you do not have a high temperature. The NHS says to ask for an urgent GP appointment or get help from NHS 111 if a high temperature you have been treating at home is not getting better or is getting worse.

Sepsis is the emergency to know about. The NHS says it is a serious reaction to an infection that can be life-threatening, develops very quickly and needs urgent treatment in hospital. Call 999 or go to A&E for an adult or child aged 5 or over who is breathing very fast, is confused, has slurred speech or is not making sense, has blue, pale or blotchy skin, has a very high or very low temperature or is shivery, has a rash that does not fade when you press it, or has symptoms you are worried might be sepsis. Do not drive to A&E.

For a baby or child under 5, the NHS adds signs such as grunting or fast breathing, a weak, high-pitched or continuous cry, being difficult to wake or more sleepy than usual, not feeding, or not having had a pee for the last 12 hours. It advises trusting your instincts: someone with sepsis might not have every symptom listed.

• Call 999 or go to A&E: signs of sepsis, a rash that does not fade when pressed, or severe breathing difficulty.

• Call NHS 111 or ask for an urgent GP appointment: a high temperature that is not improving or is getting worse; or if you have had sepsis in the past 12 months and have symptoms of an infection.

• See a pharmacist or GP: persistent or worsening symptoms of a known infection, or symptoms of a skin infection such as impetigo.

Many infections are recognised from your symptoms and an examination. The NHS gives an example: for impetigo, a pharmacist or GP checks that symptoms are not caused by a more serious skin infection such as cellulitis. Where the cause is unclear or the infection is persistent, recurrent or widespread, doctors may take a swab for the laboratory; the NHS says a GP may swab the skin or inside the nose for recurrent impetigo.

More serious or unusual infections can need blood tests, samples from the site of infection and specialist input. The details depend on the infection, so the specific condition guides are the best place to read about tests.

Treatment depends on the cause. The NHS says antibiotics work by killing bacteria or preventing them from spreading, but they do not work for everything, and antibiotics are no longer routinely used to treat chest infections, ear infections in children or sore throats. It says antibiotics may be used for bacterial infections that are unlikely to clear without them, could take too long to clear, carry a risk of more serious complications or could infect others.

You may still be infectious after starting antibiotics. The NHS says it can take between 48 hours and 14 days, depending on the infection and how it is treated, to stop being infectious, and to ask a GP or pharmacist for advice. Take antibiotics as directed, never share them, and note that taking antibiotics when you do not need them can mean they will not work for you in the future.

Sepsis is treated in hospital with antibiotics, oxygen to help with breathing and fluids given into a vein, sometimes in a specialist care unit.

The NHS says overuse of antibiotics has made them less effective and has led to the emergence of "superbugs", strains of bacteria that are resistant to many antibiotics, including MRSA, Clostridium difficile and the bacteria that cause multi-drug-resistant tuberculosis. The biggest worry is that new strains may emerge that cannot be treated by any existing antibiotics.

This is why doctors and pharmacists sometimes advise against antibiotics for mild illness, even though many people expect them. Following their advice is part of protecting your own and others' future treatment options.

The NHS says to wash your hands for around 20 seconds, with soap and water, after using the toilet or changing a nappy, before and after handling raw food, before eating, after blowing your nose, sneezing or coughing, before and after treating a cut or wound, and after touching animals. If soap and water are not available, use alcohol-based hand sanitiser if available.

Vaccination is the other pillar. The NHS vaccination schedule starts with the 6-in-1, rotavirus and MenB vaccines at 8 weeks, and continues with MMRV, pneumococcal, children's flu, HPV and teenage boosters, then flu, pneumococcal and shingles vaccines for older adults, and RSV and COVID-19 vaccines from 75. Some vaccines are offered to people at higher risk because of long-term conditions. If you or your child have missed a vaccine, the NHS says to contact your GP surgery to catch up.

The NHS says that during pregnancy the immune system is weakened to protect the pregnancy, which can make you less able to fight off infections such as flu. Pregnant women are more likely to get flu complications such as pneumonia and more likely to be admitted to hospital. The inactivated flu vaccine and the whooping cough vaccine are recommended in pregnancy, and the RSV vaccine is offered from around 28 weeks to protect the baby in its first 6 months.

The NHS says whooping cough vaccine is usually given at around 20 weeks (it can be given from 16 weeks) and ideally before 32 weeks, and that live vaccines such as MMR, BCG, oral typhoid and yellow fever are usually delayed until after the baby is born, though a live vaccine may sometimes be used if the risk of infection is greater than the risk of the vaccine. Tetanus vaccine is safe in pregnancy if necessary.

It also says that when you are pregnant it is best to avoid travel to areas where travel vaccinations are required, and to talk to a midwife or GP if you cannot. If you are pregnant and think you have an infection, mention the pregnancy to a pharmacist, GP or NHS 111 so that advice and any medicines suit you.

Some infections must be reported to public health authorities. GOV.UK says registered medical practitioners must report a notifiable disease to the UK Health Security Agency (UKHSA), by law reporting any suspicion without waiting for laboratory confirmation. Urgent notifiable diseases must be reported by telephone to the local UKHSA health protection team within 24 hours, and all cases are reported online within 3 days.

The list includes acute meningitis, measles, tuberculosis, whooping cough, food poisoning, mumps, rubella, scarlet fever, malaria, hepatitis, COVID-19 and chickenpox, among others. This reporting is done by clinicians on your behalf; you do not need to do anything.

Most infections are managed through a pharmacy, GP or NHS 111 service, with hospital care for serious illness such as sepsis, which the NHS says is treated in hospital. Infection specialists in hospitals handle complex, unusual or imported infections and recurrent problems.

Private GP and travel clinics offer routine and travel vaccinations that are not covered by the NHS schedule, as well as private tests. NHS vaccines are free for those who qualify under the schedule, so check the NHS list before paying for a vaccine that may already be available to you.