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Flu: Overview

Created: ; Last Update: November 7, 2019; Next update: 2022.


People often talk about having the flu (influenza) when they come down with a cold. Yet the course of these two illnesses is very different and they also typically have different signs and symptoms.

Colds are much more common than the flu. The main difference is that colds start gradually, whereas the viruses that cause the flu strike quickly and cause more severe symptoms – even in people who are otherwise healthy. The flu makes you feel very ill quite suddenly.

Colds don't usually cause any serious harm, and are often over within a week, with or without treatment. It is a good idea to see a doctor if you have the flu, and it may take some time before you fully recover.

Colds and the flu do have one thing in common, though: The treatment for both focuses on relieving the symptoms. The only medicines that target the flu viruses directly can at most slightly reduce the time you are ill. But a number of things can be done to avoid infection in the first place.


Many symptoms of “real” flu are similar to those of a common cold. They might include fever, a headache, joint pain, and a stuffy or runny nose. But flu usually affects the entire body rather than just the airways. It typically starts suddenly with very severe symptoms. These usually improve a lot within one week. Coughing and exhaustion may last longer, though.

Flu symptoms include:

  • A fever between 38°C and 40°C (about 100-104°F) or higher
  • Muscle and joint pain throughout the entire body (myalgia and arthralgia)
  • Headache
  • Severe fatigue and a general feeling of being very ill
  • Dry cough
  • Stuffy and/or runny nose
  • Loss of appetite
  • Extreme tiredness

Babies or toddlers may also have stomach and bowel trouble like nausea or vomiting.

If you think you might have the flu, it's a good idea to go to the doctor. This is especially important if you already have other medical conditions – such as a chronic lung disease or diabetes – that increase the risk of developing complications. It is also a good idea to seek medical advice if you have the flu and are in close contact with people who are at high risk.


The flu is caused by viruses. Viruses are microscopically small germs – even smaller than bacteria. After entering the body, they multiply very quickly. The body’s immune system needs some time to make antibodies to fight the viral infection.

There are hundreds of flu viruses, belonging to different groups. The influenza A and influenza B viruses are the most dangerous. If you are infected with a particular virus, you will also be immune to that virus once you are better again (in other words, you won't catch it again). But because the flu viruses are always changing (mutating), completely new types of viruses can appear each year. That's why you won't develop lasting protection after having the flu.


Sinusitis (an inflammation of the sinus cavities) is one very common complication of the flu. Here the cavities surrounding the nose fill with an infectious fluid. Common symptoms of sinusitis include a headache and a stuffy nose.

Babies and young children often develop a middle ear infection when they have any kind of respiratory tract infection.

In rare cases, the flu can become more serious and lead to complications like pneumonia. The chances of this happening are especially high in people with a weakened immune system, such as babies and toddlers, people over the age of 60, or people who have a disease affecting their lungs or immune system. Very severe flu can become life-threatening.


Flu and cold viruses are spread by means of droplet infection: when someone who is infected coughs or sneezes, droplets containing the viruses are released into the air. Viruses also get onto the tissue and your hands when you blow your nose. These viruses can then pass to other people or objects. They are easily spread from person to person wherever a lot of people touch the same objects, like doorknobs or handrails on the subway or a bus. Cold and flu viruses are also more likely to spread if people come into direct contact with each other, for instance by shaking hands or hugging.

The most effective way to protect yourself and others from these viruses is to stop them from spreading – for example, by washing your hands frequently and throwing away used tissues.

Even after the symptoms have gone away, you are still contagious for up to one week. So it's a good idea to avoid contact with others during this period, perhaps by working from home if possible.

A flu vaccination can also help protect you from flu viruses.


Many people turn to home remedies like chicken broth and herbal teas to relieve flu symptoms. Getting plenty to drink is also considered to be important. But there's no scientific proof that any of these things helps speed up recovery. So you don't necessarily have to use any of these remedies or drink more fluids than you feel like drinking.

A number of freely available products like vitamin supplements or inhalation devices are marketed for the treatment of colds, coughs and flus. But there's no compelling evidence that they're effective against the flu. Painkillers such as acetaminophen (paracetamol), acetylsalicylic acid (ASA, the drug in medicines like Aspirin) and ibuprofen can relieve pain and reduce the fever. Children and teenagers shouldn't use acetylsalicylic acid, though, because it can cause a rare but dangerous side effect called Reye’s syndrome in these age groups.

In addition to these medications that you can get without a prescription, special flu medication is also available. The most common of these in Germany is oseltamivir (trade name: Tamiflu). Oseltamivir is prescription-only and needs to be taken within two days after the flu starts. It doesn't have any effect on the course of the flu if you have already had symptoms for longer than that. Some research suggests that oseltamivir could reduce the duration of the flu by up to one day. But it often causes nausea and vomiting. It's not clear whether it helps to prevent related illnesses and complications.

Some people think that antibiotics will help in the treatment of flu. But antibiotics are only effective against bacteria, not against viruses. So antibiotics only help if you have a bacterial infection of the airways as well as a viral infection. Unless there are signs of bacterial infection, there's no point in taking antibiotics.

Further information

When people are ill or need medical advice, they usually go to see their family doctor first. Read about how to find the right doctor, how to prepare for the appointment and what to remember.


© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK279459


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