The lungs are particularly vulnerable to viral attack because droplet infection is one of the major routes onto the body. A sneeze from an infected person can create an aerosol that contains millions of suspended viral particles. These can travel alarming distances and be inhaled by people many metres away.
Most viral infections we catch in this way affect the nose, throat and pharynx (voicebox), causing upper respiratory tract infections. A few viruses do travel deeper into the lung tissue, causing damage to the bronchioles and alveoli. Some viral lung infections result in viral pneumonia, which can be caused by several different viral infections including influenza strains, the respiratory syncytial virus and the virus that causes severe acute respiratory syndrome (SARS).
What causes viral pneumonia?
Pneumonia is an infection of the lungs that causes general inflammation and the production of a large quantity of fluid as part of the body’s immune response. This fluid can fill the delicate alveoli, causing consolidation of lung tissue, which can be rapidly fatal.
Infections with bacteria, fungi and viruses can all cause pneumonia but, fortunately, most viruses remain in the upper respiratory tract. Those that can spread down to the lungs, causing a much more serious illness include:
Some strains of influenza (flu) (see below)
Respiratory syncytial virus (RSV) (see below)
The SARS corona virus (see below)
Viruses from the herpes family: Herpes simplex 1 and herpes simplex 2 can both cause viral pneumonia in babies shortly after birth
Varicella zoster: infection with the virus that causes chicken pox can lead to lung complications
Adenoviruses, cytomegalovirus and parainfluenza viruses: three classes of virus that usually cause the common cold, but that can give rise to viral pneumonia in people whose immune system is weak because of, for example, HIV infection. This type of viral pneumonia can also affect babies and very young children.
What are the symptoms of viral pneumonia?
In contrast with bacterial pneumonia, which is usually extremely serious and causes severe illness within days, viral pneumonia is not as dramatic. It usually causes a significant fever and you will feel very unwell, with extreme tiredness, aching and a cough. This is usually not productive, although you may cough up some clear or white mucus.
In some cases, these symptoms last for weeks, requiring rest and recovery until the body’s immune system fights off the virus. More serious cases of viral pneumonia can require supportive hospital treatment and the danger signs that suggest this is necessary include:
Producing blood when you cough
Feeling very breathless, even when resting
Having pains in your chest that become severe
Not being able to get up because you are so weak.
Influenza and viral pneumonia
Any strain of flu has the capacity to cause viral pneumonia but individual strains can affect the lung tissue more than others. The worst strain of flu known for its ability to kill because of severe lung damage was the 1918 flu virus that swept the world at the end of World War I, killing an estimated 60-100 million people worldwide.
Many of the dead were young, fit adults and the reason they died was that the flu strain responsible produced an enzyme called a haemaglutinin, which bound specifically to human cells. This stimulated the body to produce an overwhelming immune response that destroyed the body’s own lung tissue. Such a serious impact on the lungs was frequently fatal and victims of the 1918 epidemic could go from being fine to coughing up blood and dangerously ill just hours later.
Two current strains of flu, swine flu and avian flu (bird flu), may also have gained molecules that make them much more likely to cause viral pneumonia, which is why governments have been so keen to develop effective vaccines in recent years.
Respiratory syncytial virus as a cause of viral pneumonia
Respiratory syncytial virus (RSV) usually causes no more than a head cold in fit, healthy adults or older children but it can be more dangerous in babies, particularly those who are premature or who already have lung problems. This virus is a major cause of viral pneumonia in small babies, and it tends to be more serious in the autumn and winter, when there is usually an epidemic that spreads throughout large regions both within countries and between countries.
Diagnosing a respiratory syncytial virus is necessary if a baby is showing signs of becoming very ill; it is possible to detect RSV in nasal mucus and then to give anti-RSV antibodies to help them recover more quickly.
SARS and viral pneumonia
Severe acute respiratory syndrome was first recognised and described in 2003, after the first cases were reported in Hong Kong in 2002.
SARS made global news because the virus was so infectious that world governments were concerned that a pandemic (an epidemic that covers the entire planet) was a possibility. Luckily, this never happened but by the time the panic had died down, there had been 8000 reported cases, causing 750 deaths. The death rate is around 10%, rising to around 50% in the over 65s. The outbreak was contained and there have been no new reported cases since June 2003.
The most common symptoms of SARS are:
Fever (38oC or higher)
Chills and shaking
Less common symptoms include nausea and vomiting, diarrhoea, having a very sore throat, a runny nose and feeling dizzy.
Tests that diagnose SARS and distinguish it from other viral infections that cause the same symptoms of viral pneumonia include:
How is viral pneumonia treated?
When viral pneumonia is caused by the SARS coronavirus, the swine flu virus or some parainfluenza strains, the only treatment available is giving fluids and supportive care, sometimes in hospital. Other viral pneumonias respond to anti-viral drugs:
Oseltamivir and zanamivir can be used to treat viral pneumonia caused by influenza strains A and B (except swine flu, H1N1)
Ribavirin is used to treat babies with respiratory syncytial virus, as well as anti-RSV antibodies
Aciclovir can be effective against viral pneumonias that follow chicken pox or herpes infection
Ganciclovir is used in cases of cytomegalovirus infection in the lungs.