Inflammation of the alveoli (air sacs) of the lungs, usually resulting from infection
In pneumonia, some of the alveoli (air sacs) in the lungs become inflamed and fill with white blood cells and secretions. As a result, it is more difficult for oxygen to pass across the walls of the alveoli into the bloodstream. Usually, only part of one lung is affected, but, in some severe cases, pneumonia affects both lungs and can be life-threatening.
The cause of the inflammation is usually a bacterial infection, but other organisms, including viruses, protozoa, and fungi, may also cause pneumonia. Less commonly, inhaling certain substances, such as chemicals or vomit, causes inflammation, which may lead to a serious condition called acute respiratory distress syndrome.
In the UK, pneumonia affects about 11 in 1,000 adults each year, and most people with the disorder recover completely. However, it can still be fatal, particularly in vulnerable groups such as the elderly, infants, and those with another illness, and in 2007 it resulted in about 28,000 deaths in England and Wales. Some forms of pneumonia are becoming more difficult to treat due to the increasing resistance to antibiotics of some of the organisms responsible for the disease. For this reason, pneumonia is now one of the most common fatal infections acquired in hospital.
Infants, elderly people, and people who are already seriously ill or have a long-term disease, such as diabetes mellitus, are at the greatest risk of developing pneumonia. Other people more likely to develop pneumonia are those who have lowered immunity as a result of a serious disease such as AIDS (see HIV infection and AIDS). Impaired immunity can also occur during treatment with immunosuppressant drugs or chemotherapy. People who smoke, abuse alcohol, or who are malnourished are also at increased risk of pneumonia.
Most cases of pneumonia in adults are caused by infection with a bacterium, most commonly Streptococcus pneumoniae. This type of pneumonia (sometimes called pneumococcal pneumonia) may develop as a complication of a viral infection in the upper respiratory tract, such as a common cold. Other common causes of bacterial pneumonia in healthy adults include infection with the bacteria Haemophilus influenzae and Mycoplasma pneumoniae.
The bacterium known as Legionella pneumophila causes a form of pneumonia called Legionnaires’ disease, which can be spread through air-conditioning systems. Legionnaires’ disease may also lead to liver and kidney disorders.
Pneumonia caused by the bacterium Staphylococcus aureus usually affects people who are already in hospital with another illness, particularly very young children and the elderly. This type of pneumonia can also develop as a serious complication of influenza. Other causes of pneumonia that are acquired in hospital include infection with klebsiella and pseudomonas bacteria. Viral causes of pneumonia include the organisms that are responsible for influenza and chickenpox.
In some cases, pneumonia is due to other organisms, such as fungi and protozoa. These infections tend to be rare and mild when they occur in otherwise healthy adults but are more common and potentially more serious in those with reduced immunity. For example, Pneumocystis jiroveci may live harmlessly in healthy lungs but can cause severe pneumonia in people who have AIDS (see Pneumocystis infection).
A rare type of pneumonia, known as aspiration pneumonia, may be caused by the accidental inhalation of vomit. Aspiration pneumonia is most likely to occur in people who have no cough reflex, often because they are unconscious after consuming an excessive amount of alcohol, taking a drug overdose, or sustaining a head injury.
Bacterial pneumonia usually has a rapid onset, and severe symptoms generally develop within a few hours. You may experience the following symptoms:
Cough that may produce bloody or rust-coloured sputum.
Chest pain that becomes worse when you inhale.
Shortness of breath at rest.
High fever, delirium, or confusion.
When pneumonia is caused by organisms other than bacteria, it produces less specific symptoms that may develop gradually. You may feel generally unwell for several days and develop a fever, and you may lose your appetite. Coughing and shortness of breath may be the only respiratory symptoms.
The symptoms associated with all types of pneumonia are often less obvious in infants, children, and elderly people. Infants may initially vomit and can develop a high fever that may cause a convulsion. Elderly people may have no respiratory symptoms but often become progressively confused.
Inflammation may spread from the lungs to the pleura (the membrane that separates the lungs from the chest wall), causing pleurisy. Fluid may accumulate between the two pleura (see Pleural effusion), compressing the under-lying lung and making breathing difficult.
In severe cases, the microorganism that initially caused the infection may enter the bloodstream, leading to blood poisoning (see Septicaemia). In some vulnerable people, such as young children, elderly people, or those with a weakened immune system, the inflammation caused by pneumonia may spread widely into the lung tissue and result in respiratory failure, which is a life-threatening condition.
If your doctor suspects pneumonia, the diagnosis may be confirmed by a chest X-ray, which will show the extent of infection in the lung. A sample of sputum may be collected and tested to identify the organism that has caused the infection. The doctor may also arrange for you to have blood tests to help to reach a precise diagnosis.
If you are otherwise healthy and have a mild form of pneumonia, you can probably be treated at home. Painkillers should help to reduce fever and chest pain. If a bacterial infection is the cause of the pneumonia, your doctor will treat it with antibiotics. If pneumonia is caused by a fungal infection, antifungal drugs may be prescribed. No drug treatment is usually needed for mild viral pneumonia.
Hospital treatment will probably be needed in severe cases of pneumonia and also for infants, children, elderly people, and people whose immune systems are suppressed. In all these cases, drug treatment is essentially the same as for people treated at home. Severe pneumonia caused by a viral infection, such as infection by the varicella zoster virus that causes chickenpox, may be treated with oral or intravenous aciclovir (see Antiviral drugs).
If the oxygen levels in your blood are low or if you are short of breath, you will be given oxygen through a face mask. Less commonly, mechanical ventilation in an intensive therapy unit may be required. You may have chest physiotherapy regularly while you are in hospital to help to loosen excess mucus in the airways so that it can be coughed up as sputum.
Young people who are in good health are generally able to recover from most types of pneumonia within 2–3 weeks, and there is no permanent damage to the lung tissue. Recovery from bacterial pneumonia usually begins within a few hours of starting treatment with antibiotics. However, some severe types of pneumonia, such as Legionnaires’ disease, may be fatal, especially in people whose immune systems are weakened.
Vaccination against pneumococcus (which is part of the routine childhood immunization programme) and influenza can help to prevent pneumonia, as can stopping smoking. Good personal hygiene can help to prevent spreading the causative organism.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.