Pneumonia is not a single disease entity. It is a term used to describe a generalised inflammation of the lung that can be caused by a number of different infectious organisms, mostly a variety of bacteria such as Streptococcus, Staphylococcus, Pseudomonas, Haemophilus, Chlamydia and Mycoplasma, as well as several viruses, and certain fungi and protozoans.

The most common causal pathogen is Streptococcus pneumoniae (also known as ‘pneumococcus’). These anaerobic (thriving without oxygen) bacteria are the cause of 80 per cent of cases of lobar pneumonia.

Staphylococcus aureus, bacteria commonly carried on the skin or in the nose of healthy people, can also trigger pneumonia. This is a particular problem in hospitals, where an antibiotic-resistant form of these bacteria - methicillin-resistant Staphylococcus aureus, or MRSA - is now endemic.

The often fatal legionnaire’s disease, or legionellosis, is a form of bacterial pneumonia caused by Legionella pneumophila, microorganisms that are ubiquitous in the environment. The disease can be caught from contaminated air-conditioning systems, or whirlpool or hot-spring spas (Lancet, 1996; 347: 494-9). Hot-spring spas can also be contaminated with other pneumonia-causing gram-negative bacteria (Epidemiol Infect, 1991; 107: 373-81).

Viral pneumonia, which can be caused by a simple cold, a bout of influenza or chickenpox infection, is generally less virulent.

But how do you know when you’ve got pneumonia, and not just a cold or flu?

First of all, pneumonia is nothing like a cold as it has no effects on the nose, so no nasal symptoms. But what symptoms it does produce are rather similar to those of influenza - chills, a high temperature and a sometimes bloody, sputum-laden cough.

However, the classic pneumonia symptom is a shortness of breath or unusually rapid breathing, caused by fluid collecting in and blocking the alveoli (air sacs) in the lungs. There may be pain in the chest and even stomach, too, and the nails may turn white. Getting a full diagnosis normally requires laboratory tests of the sputum and/or a chest X-ray.

Who is at risk of pneumonia? Anyone with heart disease (particularly congestive heart failure), diabetes or impaired immunity (such as people with AIDS or those undergoing chemotherapy) as well as heavy drinkers are particularly vulnerable. In hospital, any surgery to the chest can bring on the condition as can intensive care units (see main story). The very old, who may be on artificial feeding, are at risk of breathing in either food particles or throat bacteria, both of which can trigger pneumonia (Drugs Aging, 1994; 4: 21-33). Young children, particularly malnourished ones, are also at risk.