Fact Sheets
AIRBORNE INFECTIONS
What is airborne transmission?
Airborne transmission of disease occurs when viruses or bacteria travel on dust particles and on small respiratory droplets that become aerosolised when people sneeze, cough, laugh or exhale. In addition water droplets aerosolised through air conditioning units may also spread infections. Aerosolised droplets hang in the air and are able to travel considerable distances on air currents.
With airborne transmission, direct contact with someone who is infected is not necessary to become ill. The amount of exposure necessary varies from disease to disease. Many airborne pathogens (microbial agents capable of causing disease) are adapted to spreading in indoor environments, where the temperature, humidity and protection from sunlight protect them in their exposed and vulnerable period when they transmit from one person to the next.
A number of common infections can spread by airborne transmission, including:
Common Cold
The common cold is caused by a large number of different types of viruses. They all result in similar symptoms, sneezing, runny nose, sore throat, cough with or without a low grade fever, muscle aches and malaise.
It has been estimated that about 30% of common colds are acquired in the workplace, 18% in the home, 1% on public transport and the in remaining 51% of infections it is uncertain where the infection was acquired. Sneezing and nose blowing provide the overwhelming majority of aerosolised viruses, while the risk of contracting a cold from someone who is coughing is relatively minor. The risk of getting infected from talking to an infected person is minimal.
It is likely that transmission of cold viruses via the hands (touching a surface sneezed or coughed on by a person with a cold) and then toughing the face (especially the eyes or nose) is also significant.
Influenza
This serious infection is caused by a family of viruses, the influenza viruses. It is characterised by marked lack of energy, fever, headache, muscle aches and cough, which last for 2 – 5 days. Full recovery usually occurs, but marked fatigue may last for some weeks. Occasionally, complications such as pneumonia or bronchiolitis may develop, and be life threatening. Deaths from influenza are more likely in the very old, the very young, in individuals with chronic lung or heart disease, pregnant women and individuals who have disorders of the immune system.
The commonest way of catching flu is through inhalation of droplets from coughs and sneezes. It is quite contagious, and has a short incubation period (2-3 days). Individuals are infectious for 24 hours before they develop symptoms of influenza. Flu tends to occur in epidemics, spreading to most individuals who do not have immunity.
Influenza vaccine provides safe, relatively effective protection from the major strains of flu which are likely to occur in a particular season. Vaccination is required annually.
Chicken pox
Chicken is one of the classic “childhood” diseases. However significant numbers of people reach adulthood without having had chicken pox. An attack in childhood provides lifetime immunity to chicken pox. Adults and older adolescents are usually sicker with chicken pox than younger children are. In individuals with immune deficiencies, or who are receiving treatment with corticosteroids, the infection may be life threatening.
Chicken pox is highly infectious. People with chicken pox become infectious 24 - 48 hours before they develop the typical rash. They remain infectious while any uncrusted blisters remain (usually for about a week). The virus is spread through direct contact with blisters, and via the air. It is so infectious that a non-immune person can be infected from aerosol spread from an infected individual several metres away.
Chicken pox is a vaccine preventable disease. It is 98% effective against moderate and severe illness and about 90% effective against moderate or severe disease.
Legionella
Legionella pneumophila is the bacteria responsible for Legionnaire's disease, a severe pneumonia. It is also responsible for a spectrum of less severe respiratory infections. This bacterium occurs in water sources with a wide range of temperatures, pH and dissolved oxygen contents. Despite chlorination, the bacteria proliferate in cooling towers and water systems whenever conditions are favourable. Inhalation of aerosols generated by air conditioning, nebulisers, humidifiers and showerheads is the major mode of transmission. Person to person spread does not occur. Other Legionella species are found in soil, and their presence in potting mix is thought to be responsible for a number of cases of pneumonia occurring in the community.
Outbreaks of pneumonia have been associated with contamination of water cooling towers in large buildings, with spread of the bacteria through air conditioning systems. The disease most often affects the elderly and people with underlying illnesses such as cancer or those with a lowered immune system. It rarely occurs in otherwise healthy people.
Apart from vigilant cooling tower hygiene, there is no means of preventing this disease. Antibiotic treatment is effective in the majority of cases, once the diagnosis is recognised.
Tuberculosis
Tuberculosis or TB is caused by the bacteria Mycobacterium tuberculosis. The lung is most often affected, although infection of the kidneys, bones and joints, and lymph glands also occurs. The causative organism is slow growing, but may persist for years in conditions of limited sunlight, cool and damp.
TB is infectious, and is spread through the air from one person to another. Infection of the throat and lung are most commonly associated with spread of infection. A person with active TB exhales bacteria into the air when he or she coughs and sneezes. Infection with TB after a single exposure is unusual in healthy individuals, and people with TB are most likely to spread it to people they spend time with every day – family, friends and co-workers. Tuberculosis infection is known to have occurred during prolonged air travel when there is a passenger with open TB. The re-circulation of a significant volume of the air on long haul flights is thought to be the reason for this.
Most people infected with TB are able to prevent the bacteria from growing. The bacteria become inactive, but there is always a risk of re-activation at a later time.
Active TB occurs when the immune system is unable to stop the bacteria from growing. This may occur soon after infection, or may occur later in life if the immune system becomes impaired. TB in the ling causes prolonged cough with sputum, coughing up blood, weight loss, fever, night sweats, chills and poor appetite.
TB can only be spread when the individual is actively unwell and coughing up sputum containing many bacteria. Treatment with appropriate antibiotics usually renders an individual non-infectious within 2 weeks of starting treatment. Treatment usually consists of 3 or 4 drugs taken for a period of 4-6 months.
Australia has a low incidence of TB. Approximately 80% of reported cases occur in migrants to Australia. The incidence is higher in indigenous Australians than in non-indigenous Australians.
