Influenza: Causes, Types, Symptoms and Treatment

Influenza, simply known as the flu, is a viral infectious respiratory disease. It is caused by several specific viruses known as influenza viruses. Influenza is different from the common cold in that it is caused by different types of virus strains and can produce mild to severe symptoms that open the way to potentially life-threatening complications and secondary infections. Categories at risk for complications from the flu include children, the elderly, anyone with immune system deficiencies or chronic illnesses. Because it is caused by a virus, influenza does not respond to antibiotics and essentially has no cure. Antiviral medication may be used to reduce the severity of symptoms and shorten the duration of the infection, while vaccines together with good hygiene and good nutrition are recommend as the best way to prevent getting the flu.

Influenza is a seasonal respiratory infection, meaning it is more likely to occur in some seasons rather than others. More exactly, flu outbreaks are most common in fall and winter in temperate climates and during the rainy season in tropical climates. For temperate climates, the flu season in the Northern hemisphere does not coincide with the flu season in the Southern hemisphere since winter occurs at different times of the year. This also means the flu season this year will be caused by different influenza virus subtypes in the two hemispheres, requiring different vaccine formulations as well.


The reason why the flu is seasonal is because virus transmission is favored by specific weather conditions. Studies show cold temperatures better conserve the flu virus on surfaces while the dry air favors its transmission as well as damages nasal mucous membranes, allowing influenza viruses to cause an infection more easily. Lack of sunshine and low vitamin D intake during winter as well as the increase in time spent indoors when it’s cold outside or rainy in the tropics make  the flu more contagious. For example, the 2017-2018 flu season in the temperate regions is expected to register outbreaks from early October to late March in the Northern hemisphere and May to late September in the Southern hemisphere.

Types of Influenza (flu)

The common cold can be caused by around 200 different virus strains, the most common being rhinoviruses (see article on the Common Cold: Causes, Symptoms and Treatment). The flu on the other hand may be caused by one of three major influenza viruses and their subtypes. The relatively smaller number of infectious agents is also the reason why there are vaccines for the flu but not for the common cold. Here are the main viruses that cause the flu:

1) Influenza A. There is one species of influenza A with multiple subtypes. Not all influenza A subtypes infect humans. The strains with the potential to infect humans (as well as pigs, birds, horses or bats, depending on the case) include H1N1, H2N2, H3N2, H5N1, H7N3, H7N7, H7N9 and H9N2. For example, the H1N1 influenza virus strain caused both the Spanish flu and the more recent Swine flu. Birds are often the most common carriers of influenza A viruses that can infect humans and this particular virus is responsible for most cases of the flu.

2) Influenza B. There is also only one species of this virus, but with multiple subtypes. Influenza B has one subtype that is pathogenic to humans and others that exclusively infect seals and ferrets. It is less dangerous than influenza A viruses, partly because it mutates a lot slower which allows the human immune system to build good immunity against it.

3) Influenza C. With one species, influenza C has the potential to infect humans, but also pigs and dogs. Respiratory infections caused by this particular virus species are rare and often confined to small communities.

The 2016-2017 flu season was caused by influenza A H1N1, H3N2 and an influenza B virus strain similar to the one that also caused outbreaks in 2008 in Australia. The 2017- 2018 flu season was caused by influenza A H1N1, H3N2 and an influenza B virus strain similar to the one that caused major outbreaks in 2008. It is important to remember that these are not the exact same virus strains that caused flu outbreaks in the past, but mutated versions. This is because all viruses mutate continuously, including influenza viruses, which are known to borrow traits from one another and form different versions of the same virus subtype. Because we develop immunity to influenza strains following infection, they need to update themselves in order to be able to cause infection again.

Influenza symptoms

Typically, influenza without secondary infections lasts for about 2 weeks, from the time the infection with the virus occurs until all symptoms disappear and the person is not longer contagious. Symptoms are similar to those of the common cold, but include fever and are usually more severe. However, not everyone will show signs of an infection and not everyone will exhibit the same range of symptoms.

Influenza signs and symptoms

1) Fever. Flu fever is usually high (close to 39 degrees Celsius and higher) and may last for the first 3-4 days.
2) Light sensitivity, red, tired eyes. Fever causes eye bulbs to hurt or feel heavy.
3) Chills and shivering. Some people experience fever alternating with strong chills and shivering.
4) Aches and generalized muscle pain. Joints may be affected, there is aching in the bones.
5) Headaches, weakness, malaise, fatigue. General feeling of unwell.
6) Loss of appetite accompanied by nausea and vomiting.
Also common when there is a stomach bug as well.
7) Nasal congestion. Sneezing is not very common. Use of nasal decongestants is not recommended (see Side Effects of Nasal Decongestants). Sprays with saline solution work best.
8) Dry cough which may persist for weeks. Little or no mucus.
9) Chest discomfort. Wheezing, difficult breathing, yellow or green mucus may indicate a secondary infection, possibly bronchitis, pneumonia or other infections (see article on Nose mucus colors).
10) Redness and rash cause by broken capillary blood vessels.
Common in influenza A.
11) Lightheadedness, dizziness, fainting. May occur as a result of dehydration and hypoglycemia. Read more about Influenza and Fainting.

When to see a doctor?

If the fever is high (over 40 degrees Celsius) and does not break in 3-4 days, it is recommended to see a doctor. If you cannot keep fluids down and cannot eat or experience frequent vomiting, go to the hospital. If chest discomfort grows worse, you experience wheezing or difficulty breathing, it is possible to have contracted a second infection and need to see your doctor as soon as possible for treatment. If it’s a bacterial infection, your doctor will prescribe antibiotics.

Usually, complications of the flu are most common in children, the elderly, pregnant women, anyone with an immune system disorder or disease such as HIV or AIDS, cancer or those with chronic illnesses. Anyone with diabetes, anemia, cardiovascular disease, asthma or chronic lung disorders (cystic fibrosis, chronic obstructive pulmonary disease) represents a risk category. Complications from the flu may include: secondary infections such as bronchitis, pneumonia, sinus infections, ear infections and worsening of symptoms in those with exiting medical conditions.

Influenza and other respiratory viruses. What an influenza virus does is produce a range of symptoms that discourage eating and drinking fluids, with the purpose of weakening the immune system. This makes it easy for the virus to grow and infect the host. It also allows for other opportunistic pathogens to produce infection as well. For example, you can be sick with the flu and throat strep at the same time. Or get the flu and the common cold. Or develop influenza followed by pneumonia.

How does the influenza virus work? Influenza enters the body via the nose, mouth or eyes. Microscopic virus particles reach mucous membranes of the respiratory tract and bind to cells on their surface. Each virus particle enters a cell, where it takes over the nucleus and starts replicating. All copies of that virus particle take over other cells and continue to replicate themselves. Below is a great video showing how the influenza virus spreads.

How does influenza spread?

The flue can spread directly or indirectly:
1) Directly. The virus is contained in mucus and can be released by sneezing, coughing, talking or shouting. For example, when a person sneezes, they release tiny drops of fluid containing the virus. These tiny drops can enter the eyes, nose or mouth and produce infection.

2) Indirectly. Flu viruses may live in the air or on hard surfaces like door knobs, desks, computer mouse or on various objects like glasses, light switches, coins or bills, tissues. If the weather is cold, the virus may remain active for hours or days at a time. Touching infected surfaces, then touching your nose, eyes or eating without washing your hands first causes you to catch the flu. Metal and plastic surfaces in particular as well as objects that are used by many people are the most likely indirect source of infection with influenza.

Studies show there are certain factors that increase the likelihood of getting sick with the flu:
1) Low temperatures. The colder the weather, the stronger the virus and the longer it survives in the air and on surfaces. Low temperatures are also one of the reasons why influenza is most common in winter.

2) Dry air. Dry air dries out mucous membranes in the nose especially and may result in tiny breaks or tears. These make it easier for viruses to produce infection.

3) Rainy weather. In tropical regions, flu season outbreaks have been shown to be common during the rainy season. Because people are indoors most of the time, this increases chances of the virus being passed down from person to person. Other explanations include: damp weather sensitizes the body to colds and other respiratory infections and water and humidity favor virus transmission.

4) Vitamin D deficiency. Both during winter and in the rainy season when flu outbreaks are most common there is a lack of exposure to sunlight which causes a shortage of vitamin D. And since vitamin D is essential to the immune system, a resulting deficiency could make catching the flu more easier.

5) Crowded places. Going to day care, school, work, concerts, parties, public institutions or any crowded place during a flu outbreak increases your chances of getting sick too.

Treatment options and other solutions

Influenza is a viral disease and is caused by a virus which mutates continually. Every year, the virus develops new characteristics that simply cannot be predicted, hence the reason there is really no cure or treatment for the flu. Because it’s a virus, antibiotics don’t work either. At most, antiviral medication may help the immune system and improve symptoms, but since our immune system is adapted to recognizing any new pathogen in order to eliminate it, it does a better job than any medication ever will. And all it need is our cooperation which consists of:

1) Resting. Any activity uses up energy the body needs to fight infection so resting is essential.
2) Staying hydrated. Water, fruit juices, teas, soup can help prevent dehydration and fainting.
3) Eating. Good nourishment supports the immune system and provides it with the nutrients it needs to work optimally. Look for foods rich in vitamin C and A, B vitamins, iron, zinc and protein.
4) Less medication is better. Flu medication, cough syrups, nasal decongestants, antihistamines all have side effects and do very little, if anything to combat the flu.
5) Avoid caffeine, alcohol and smoking. They worsen symptoms and produce side effects.
6) Vaccination. The flu vaccine is reformulated every year to cover the most likely influenza strains, so it constitutes a good prevention, contributing to milder symptoms and faster recovery. When to get the flu vaccine? Talk to your doctor about when the flu vaccine will become available in your area and ask to be informed about side effects and adverse reactions, especially if you have existing medical conditions
7) Take measures to reduce exposure to the flu virus. This include frequent hand washing, not sharing personal objects, not drinking or eating after anyone with the flu, disinfecting surfaces and avoiding crowded places during the flu season.

This post was updated on Friday / July 31st, 2020 at 10:26 PM