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Pathology and causes of Pneumonia

Pneumonia is an inflammatory condition of a lobe or the whole lung caused by bacterial, viral and fungal infections. It may lead to pleural effusion. Prior to the invention of antibiotics, a third of pneumonia patients died. Statistics of the Department of Health revealed that more than 5,800 people died from pneumonia in 2010 in Hong Kong, taking up 13.6% of the total number of deaths in the year.

There are two types of pneumonia: community acquired pneumonia (CAP) and hospital acquired pneumonia (HAP). CAP occurs outside hospital or within 48 hours of admission while HAP 48 hours after admission or within 14 days after hospital discharge.


Our lungs prevent invasion of pathogens. If microorganisms survive in the lungs, they will breed fast and cause pneumonia.

Pneumonia carriers may spread the disease through coughing, sneezing, or having direct contact with others. Contacts with objects contaminated by their nasal discharge or sputum may also cause the disease. Pneumonia can also be caused by the presence of secretions from the oropharynx which contain bacteria or viruses. When these secretions get into our lungs, we will cough and try to get rid of them and the immune system will fight against the invading germs. But for young children and the elderly, their body is not ready to do so or cannot do so. They may get infected as their immune system is not strong enough to protect them. Patients with leukaemia, lymphoma, chest and lung diseases, heart disease, epilepsy, or swallowing difficulty, or those who have had a stroke, or people who abuse alcohol or drugs are at a higher risk in getting pneumonia. Upper respiratory tract infections not properly treated (such as using cough medicine without seeing a doctor) may also lead to accumulation of sputum and subsequent bacteria, which in turn will cause pneumonia.

Pneumonia may affect everyone. Those with a strong immune system may also suffer from the disease when invaded by more poisonous microorganisms. For those with a seriously compromised immune system, they may be infected as they are invaded by microorganisms that normally cause no problem. And this can be fatal.


If infected by pneumonia, patients may have symptoms like coughing, nasal discharge, and sore throat, which are similar to those of upper respiratory tract infections. They may also experience shortness of breath, fever and sometimes productive cough or blood tinged sputum. If breathing is affected, the level of blood oxygen may drop, leading to breathlessness, pale skin or cyanosis. If the infection occurs around the pleura, it may cause pleuritic chest pain, which may be more severe when taking deep breath or coughing.

Young children affected by pneumonia may only experience fever and mild cough; they may have no other obvious symptoms.

Common types of pneumonia:

Bacterial pneumonia

Streptococcus pneumoniae is the most common bacteria causing the disease. It is found in the nose and throat of healthy people, particularly in children. Streptococcus pneumoniae is the cause of various forms of invasive pneumococcal diseases (IPD) and a common causative agent for middle ear infection. It can also cause meningitis and septicaemia (caused by infections of blood stream). These infections can be life-threatening.

Young children, the elderly, people with a history of IPD, patients with chronic illnesses, those with weakened immunity such as cancer or HIV/AIDS patients, and those with their spleen removed are at high risk. These patients may experience sudden onsets of symptom, shivering, fever, chest pain which may get worse when taking deep breaths and coughing. They may have rusty sputum, general discomfort, poor appetite, muscle pain and headache.

Antibiotics commonly used to treat pneumococcal infection are:

  • Penicillin
  • Erythromycin
  • Clarithromycin
  • Azithromycin
  • Amoxicillin

Penicillin was the drug to treat pneumococcal infections before. But its general use as a broad-spectrum antibiotic has given rise to antibiotic resistant pneumococcus. The drug can still be used; but the sensitivity of the bacteria must be identified beforehand.

These antibiotics may have side effects including diarrhea, vomiting, dizziness, and headache.

The emergence of antibiotic resistant pneumococcus has made treatment more difficult. But we can receive vaccines for better protection. These vaccines are safe and effective. The Centre for Health Protection recommends the high-risk group and children aged below two to receive a pneumococcal vaccine. The type of vaccines to be received depends on the recipient's age. Pneumococcal conjugate vaccines are for children while pneumococcal polysaccharide vaccine are for both children and adults. Newborns should receive a standard 3-dose primary series at 2nd, 4th and 6th months of age and a booster dose between the 12th and 15th months. Children aged 12 months or above who have never received any pneumococcal vaccine will receive only one dose. Pneumococcal vaccine is safe to use and causes no serious reactions in children. Occasionally there may be mild fever, or slight tenderness or swelling around the injection site but these will gradually subside in a few days. There are more than 90 strains of pneumococcus. For details on the strains covered in the vaccine, please talk to your doctor.

Legionnaires’ disease 

In Hong Kong, Legionnaires’ disease (LD) is a statutory notifiable infectious disease. It is caused by a type of bacteria called Legionella, which are found in various environmental settings and aqueous environment such as water tanks, hot and cold water systems, cooling towers, whirlpool, air-conditioners, medical equipment and even plant soil. They grow well in warm water (20 – 45°C). Men are two to three times more susceptible than women; people over 50 years old, smokers, drinkers and patients with chronic medical conditions, those with weaker immunity, and those using steroids or immunosuppressant drugs may be affected more easily.

Symptoms are fever, dry cough, difficulty in breathing, fatigue, headache, muscle pain, abdominal pain, diarrhea, and chest pain; in severe cases, there may be neurological symptoms (e.g. confusion); it may result in respiratory failure or even death.

Urine antigen testing can be used to detect the presence of antigens in an infected person’s urine. LD can be treated by antibiotics:

  • Erythromycin
  • Clarithromycin
  • Azithromycin
  • Fluoroquinolones

There is no vaccine yet. To prevent LD, we must start with cleaning up our living environment. It is most important to operate and maintain properly designed water supply systems to prevent the breeding of Legionella. Drain and clean water tanks of buildings at least quarterly. Drain the infrequently used water outlets and stagnant points of pipe work regularly. Remove strainers for cleaning and replace filters in water purifiers regularly. Persons with weakened immunity should use sterile, distilled, or boiled water for drinking, tooth brushing and mouth rinsing, and avoid using mist-generating devices.

Viral pneumonia

It can be caused by adenovirus, rhinovirus, influenza virus, respiratory syncytial virus, or parainfluenza virus. It is a common disease among children and those affected by cardiopulmonary disease.

Viral pneumonia is not a severe disease. Our immune system can fight off the virus and the symptoms will subside gradually. The pneumonia caused by influenza virus (such as H1N1) can be very serious and may lead to respiratory failure and death.

The symptoms of viral pneumonia and influenza are quite similar. Patients may experience fever, dry cough, headache, muscle pain and loss of strength, and difficulty in breathing.

Unless the conditions are severe or there are complications, antibiotics are not used. The goal of treatment is to relieve the symptoms.

Contaminated airborne droplets may spread viral pneumonia. To prevent the disease, it is important to stay away from crowded areas, to keep a balanced diet and have enough rest. High-risk groups should receive an influenza vaccine four to six weeks before the influenza peak season. The vaccine is 70% to 90% effective in preventing influenza and its complications. There are two types of vaccines: inactivated influenza vaccine and live attenuated influenza vaccine. Inactivated influenza vaccine may lead to side effects like local pain, swelling, fever, tiredness and muscle pain. It is very unlikely for any recipients to have complications or severe allergic reactions. The common side effects of live attenuated influenza vaccine are nasal discharge and congestion, fever and sore throat. It is not to be used on pregnant women or those with weakened immunity. These two vaccines are intended for different groups. Please talk to your medical consultants and understand the risks involved before receiving the vaccine.

Other types of pneumonia

Pneumonia can easily affect those with lower immunity, e.g. cancer patients undergoing chemotherapy, leukaemia patients, HIV/AIDS patients, rheumatoid arthritis patients. The disease can also be caused by fungi and parasites. If not treated promptly, it can result in death. Different fungal infections are treated differently. Parasitic infection is mainly caused by Pneumocystis carinii. This type of pneumonia seldom affects people with a normal immune system. It is mainly treated by antibiotics. In severe cases, steroids are also used.