The lungs are particularly vulnerable to infection because we are constantly inhaling a variety of airborne particles that contain pathogens. All types – bacteria, fungal spores, parasites and viruses – can cause lung infections. In this article, we focus on bacterial and fungal lung infections.
How do we avoid lung infections?
The lungs have an efficient self-cleaning and defence system that consists of:
A carpet of tiny hairs, called cilia, that lines the airways. A thin layer of mucus traps inhaled particles and the cilia drive this up towards the throat, where it is swallowed
The coughing reflex. This also helps to dislodge mucus and anything causing irritation, thus preventing lung infections
White blood cells that patrol the lining of the airways. These cells engulf any foreign particles they find, destroying bacteria and any cells that are loaded with viruses.
Despite this, lung infections are common, even in healthy people. People who have disabled their defences by smoking, or those who have a weakened immune system, may experience even more frequent lung infections.
What are upper respiratory tract infections?
Infections of the nose, throat and voice box (pharynx) are classed as URTIs: upper respiratory tract infections. Strictly speaking, these are not lung infections but they can spread to the lungs if not treated. Treatment of bacterial URTIs with antibiotics is usually effective within a few days.
In vulnerable people, a bacterial URTI or a chest or lung infection can sometimes develop after a viral infection. Viruses cause a range of upper respiratory tract infections, including the common cold and flu (find out more about viral lung infections). Viruses enter cells that line the airways and multiply, causing huge numbers of cells to die in the process. This tissue damage disrupts the delicate balance of normal commensal bacteria, and pathogenic bacteria can find a way in to attack the body.
Serious bacterial lung infections
Infections that start in the upper respiratory tract can sometimes spread to the lower respiratory tract causing a secondary lung infection. It is also possible for bacteria inhaled deep into the alveoli to set up a primary lung infection. Common serious bacterial lung infections include bacterial pneumonia, tuberculosis and fungal lung infections.
The most common causes of bacterial pneumonia are:
These bacteria cause many cases of childhood pneumonia worldwide; we now have vaccines that can protect against both and there is a growing effort to immunise the millions of children who succumb to this potentially fatal lung infection every year.
Pneumonia is defined as any lung infection that involves:
Inflammation of the airways. The linings of the bronchi and bronchioles become swollen and secrete more mucus than usual. Air flow is restricted
Consolidation of the alveoli, where the air sacs fill up with fluid and become very dense and unable to take in air.
Bacterial pneumonia tends to cause symptoms to come on suddenly. You can feel fine, but then start to feel very unwell within hours. The most typical symptoms include:
Having a high temperature. Normal body temperature is 98.4oF, but someone with pneumonia can have a temperature of 104oF or 105oF. Someone with a serious bacterial lung infection may feel alternately hot and cold, and feel clammy and sweaty
Breathing very rapid and shallow breaths
A cough that produces green mucus
Pain on coughing and general chest pain
Signs of cyanosis when severe: blue lips and blue fingernails.
Pneumonia can be distinguished from other lung infections using an X-ray. Areas of consolidated tissue are very obvious. The underlying cause can be investigated by analysing the sputum (the mucus that is coughed up). It is important to treat pneumonia with a broad spectrum antibiotic as soon as possible, but then a more specific antibiotic can be used once the organism causing the lung infection has been identified.
Tuberculosis is a debilitating bacterial lung infection but can, if left untreated, spread to other parts of the body. In the UK today, cases are relatively rare, although they are on the increase due to resistant strains of bacteria emerging. In other parts of the world, particularly in developing countries where HIV is common, tuberculosis affects and kills millions of people each year.
TB is caused by a slow-growing bacterium called Mycobacterium tuberculosis. This is transmitted via inhaled droplets passed on from an infected person. Fortunately, TB is not as infectious as diseases such as colds and flu. You are unlikely to contract tuberculosis from, say, somebody coughing on you just once. Lung infection with tuberculosis requires prolonged exposure to people who are already infected.
Common situations for TB transmission include prisons, homeless shelters and foreign travel.
In its early stages, tuberculosis causes no symptoms. Later, active TB will cause:
If a lung infection that turns out to be TB is untreated, nodules of infection develop, in which bacteria are surrounded by the body’s white cells, forming granulomas. Bacteria and cells in the centre of the granulomas die, forming the tubercles that give the disease its name. This causes widespread destruction of the lung tissue, which can be fatal.
Bacteria can spread in the bloodstream to other parts of the body, commonly the lymph nodes, brain, bones and kidneys, and repeat the process seen in the lungs. This is nearly always deadly.
Treatment for tuberculosis is not straightforward. People with active TB in the UK are usually treated by a team that includes a lung infection specialist, a pulmonary expert and, for follow up, specialised health visitors. The long-term use of several antibiotics is necessary to overcome the new, resistant strains. Commonly, rifampicin and isoniazid are the first two lines of attack, used every day for six months. In addition, pyrazinamide and ethambutol are used every day for the first two months. Recovery is usually slow and it will be weeks or months before the symptoms subside.
Fungal lung infections
Fungi are organisms that generally survive by breaking down dead organic matter and, as such, they are vital to most ecosystems. Very few species are pathogenic. Fungal spores are very common and are inhaled on a regular basis. People are particularly likely to encounter fungal spores if involved in demolition, renovation of old buildings or working with soil.
Usually, fungi do not survive in the lungs and if they do, they are dealt with by the immune system. Consequently, cases of fungal lung infection are rare in the UK, and are usually only seen in people who have a weakened immune system due to HIV infection, cancers of the immune system (such as leukaemia) or immunosuppressant drugs that are given following an organ transplant. Rare cases also occur in people who have travelled abroad to countries where fungal disease is endemic.
Fungal lung infections are called mycoses. Those that result from a weakened immune system are called opportunistic mycoses and those that result from foreign travel are called endemic mycoses. The commonest fungal lung infections are caused by Aspergillus and Candida species.
Generally, fungi can cause disease in two ways: