Pneumonia is an inflammatory disease mainly caused by an infection with bacteria or viruses and rarely by fungi, but it can be caused by non-infectious agents, such as inhaling food or fluids. The disease affects the air sacs, or alveoli, of the lungs. Usual signs and symptoms vary from mild to severe. A combination of symptoms can appear, such as coughs, fever, chills, and breathing difficulties.

People who are in high risk of getting pneumonia include patients with chronic diseases, weak immune system, a history of smoking, inpatients and infants less than 2 years and people over 65 years. 

Diagnosis is often based on the symptoms and physical examination. Chest X-ray, blood tests, sputum test and pulse oximetry to measure oxygen levels in the blood may help to verify the diagnosis.

Infectious pneumonia is classified according to where the infectious agent was acquired: in the community, hospital, or healthcare centers. 

Taking vaccines is a good preventive action against pneumonia. Other methods of prevention include good hygienic practices, such as using hand sanitizers.

Antibiotics, antiviral medications and antifungal medications are used for treating pneumonia depending on the type of agent causing the infection. Generally, if pneumonia is severe, the affected patient should be hospitalized. 

Pneumonia can be a deadly disease, especially in children. The World Health Organization reported that the disease, which is most prevalent in South Asia and sub-Saharan Africa, was responsible for 16% of all global deaths of children under 5 years of age in 2015.

Pneumonia is an inflammatory disease that results from bacteria, viruses, or fungi. Aspiration pneumonia occurs when food, liquids, saliva, or vomit is breathed into the lungs. The disease affects the respiratory system, and occurs in one or both lungs. Infection in the lung leads to inflammation in the air sacs, also called alveoli. As a result of this inflammation, the air sacs become full of fluid or pus, which affects their ability to function properly.   

Pneumonia can be classified according to the causing agents into:

• Non-infectious pneumonia

This type of pneumonia is called aspiration pneumonia because it is caused by the inhalation of substances such as food, fluid and dust into the lung.

• Infectious pneumonia

Pneumonia resulting from Infectious agents includes three subtypes classified according to where the infection was contracted: 

• Community-acquired pneumonia: This is the most common type of pneumonia where the infection occurs outside the hospital or other health care services.
• Hospital-acquired pneumonia: In this type, the patient contracts the disease during a stay in the hospital. This type of pneumonia can be severe because the health status of the patient is already compromised, and the bacteria causing it may be more resistant to antibiotics. Patients in intensive care units who are using ventilators are at a higher risk of this type of pneumonia.   
• Health care- acquired pneumonia: Infections of this type occur to people who live in long-term care facilities, or who receive care in outpatient clinics. Bacteria that cause this type of pneumonia can be resistant to antibiotics.

Infectious pneumonia can also be classified according to the causing agent which can be bacterial, viral or fungal.

Pneumonia can be caused by several infectious and noninfectious agents. These include: 

• Bacterial pneumonia: Bacteria is the most common cause of pneumonia. There are many types of bacteria that can cause pneumonia, including streptococcus pneumoniae, legionella pneumophila, mycoplasma pneumoniae and chlamydia pneumoniae.

•  Viral pneumonia: Viral pneumonia can be caused by the influenza virus, common cold virus, rhinovirus, human parainfluenza virus (HPIV), and human metapneumovirus (HMPV). Usually, the symptoms of viral pneumonia are mild, but some cases may be severe enough to warrant hospitalization.   
• Fungal pneumonia: Patients with chronic diseases or weak immunity are more likely than others to have pneumonia caused by a fungal infection. The fungi that cause pneumonia can be found in soil or bird droppings and vary depending upon geographic location.
• Chemical pneumonia: This is caused by breathing in gases and small particles, such as dust or fumes. Only a small percentage of pneumonia are considered as chemical.
• Aspiration pneumonia: This is considered as a type of chemical pneumonia. It can develop after a person inhales food or liquid into the lungs, which causes inflammation. Some conditions that may put the person at risk for aspiration pneumonia include brain injury, swallowing problems, and excessive use of alcohol or drugs.

Anyone can develop pneumonia, but immunocompromised groups are more likely to have pneumonia than others, this group includes: 

• Infants who are two or younger, and old adults who are 65 or older. 
• Inpatients, especially those who are using a ventilator. 
• Patients with chronic diseases such as asthma, chronic obstructive pulmonary disease or heart disease. 
• Smokers because cigarettes damage the natural defenses of the body against the bacteria and viruses which can cause pneumonia.
• Patients with weak immune systems due to an HIV/AIDS infection, patients who use immunosuppressant drugs following an organ transplant, or those who receive chemotherapy or long-term steroids.

A lung infection causes inflammation in the alveoli. Capillaries around the inflammation site start to leak protein-rich fluid inside the inflamed alveoli, causing them less efficient in oxygen-carbon dioxide exchange. This leads to the patient suffering from oxygen deficiency while retaining potentially damaging carbon dioxide. The patient starts to breathe heavily to collect as much oxygen as possible, and blow off more carbon dioxide.

The increased production of purulent mucus is a further hindrance to efficient gas exchange in the lung. The alveoli become filled with fluid and debris which come from the large number of white blood cells being produced to fight the infection.

The severity of the symptoms of pneumonia depend on the type of agent causing the inflammation, and also depends on the age and health status of the patient. Signs and symptoms of pneumonia may include:

- Cough

- Chest pain when breathing or coughing

- Fatigue

- Fever, sweating and shaking chills.

- Nausea, vomiting or diarrhea

- Shortness of breath

- Confusion or changes in mental awareness, especially in adults age 65 and older

- Decreased body temperature, especially in adults age 65 and older and patients with compromised immune systems

Pneumonia is diagnosed by a combination of physical exams and other tests to confirm the diagnosis. Pneumonia is diagnosed by:

- Physical exam: The doctor here uses a stethoscope to examine sounds made by the lungs; bubbling or crackling sounds may indicate pneumonia. Other tests are usually done after the physical exam to confirm the diagnosis.

- Blood tests: Complete blood count is used to determine if levels of white blood cells are increased to fight the infection. Also, a blood sample can be tested to check for elevated levels of reactive protein C and distinguish the type of the microorganism causing the infection.

 - Chest X-ray: Although this test cannot identify the type of the microorganism causing the infection, it is the best test available to verify its location and extension.

- Pulse oximetry: This test measures oxygen levels in blood. Because an infected lung cannot transfer enough oxygen to the blood stream, the doctor tests a blood sample from the patient to check oxygen levels.

- Sputum test: This test is used to identify the cause of the infection. A sample of sputum or phlegm is collected from the patient to analyze it.

 In severe cases the doctor may recommend more testing that may include: 

- Computerized tomography (CT) scan: A chest CT scan is recommended if the type of infection is not clear.

 - Pleural fluid culture: This test is performed only if there is pleural effusion. A procedure called thoracentesis is conducted to collect a fluid sample by inserting a needle in the pleural area between two ribs. This test is helpful in studying the type of agent causing the infection.

 Treatment of Pneumonia

The main goal of treatment is to cure the infection and prevent complications. Treatment methods are classified according to the type of infectious agent into:

 - Bacterial pneumonia: Antibiotics are used to treat bacterial pneumonia. Identifying the type of bacteria causing the infection and choosing the correct antibiotic to treat it may take time.

 - Viral pneumonia: There are no antibiotics for viral pneumonia, so doctors prescribe antiviral medications, if possible, to control the symptoms.

- Fungal pneumonia: Fungal pneumonia is treated by identifying the type causing the infection and using the proper antifungal medication.

- Aspiration pneumonia: Treatment depends on the severity of the symptoms. Some patients may need hospitalization to assist breathing. The need of antibiotics is considered as well.

Medications, such as cough medicines and pain relievers, are usually taken by patients with any type of pneumonia. 

In some cases, pneumonia becomes severe and the patient must be hospitalized. These cases include: 

- Patients above 65 years of age

- Degradation in kidney function.

- Systolic blood pressure below 90 mm Hg, and diastolic blood pressure above 60 mm Hg.

- Rapid breathing, or need of breathing assistance.

- Low body temperature

- Irregular heartbeat

Some patients with pneumonia, especially immunocompromised patients, are exposed to having complications even if they receive proper treatment. These complications include: 

- Breathing difficulties: In severe cases of pneumonia, the patient becomes unable to breathe efficiently to have enough oxygen.

- Spread of infection: Bacteria may enter the bloodstream from the lungs and spread the infection to other organs, e.g. kidneys, which may lead to organ failure. 

- Pleural effusion: This happens when the thin space between the layers of tissue that line the lungs and chest cavity begin to collect fluids.

- Lung abscess: Pus may form in the lung cavity causing an abscess. Abscesses are treated with antibiotics, surgery or drainage through a needle or tube.

There are some practices that can help prevent pneumonia from developing:

- Vaccination: Vaccines can prevent pneumonia caused by pneumococcal bacteria or the flu virus. Nevertheless, vaccines cannot prevent all cases of infection, but vaccinated patients who develop pneumonia tend to have a slighter infection, a shorter disease period, and fewer serious complications.

Types of vaccines that can be taken: 

· Pneumococcal pneumonia vaccines.

· Influenza vaccine.

· Haemophilus influenzae type b (Hib) vaccine.

- Good hygienic practices: Washing hands frequently and using alcohol-based hand sanitizer can reduce the probability of getting an infection.

- Stopping smoking: Smoking can damage the lung’s ability to filter inhaled air and defend against germs.

- Improving immunity: Eating a healthy diet rich in vitamin C, getting enough sleep and doing physical activities can be useful in improving the body’s immunity, which may protect against pneumonia.

The prognosis of pneumonia varies according to its type: 

- Community-acquired pneumonia: Most patients who follow treatment get well within two weeks, but severe cases, and cases in immunocompromised people become life-threatening and may lead to death.

- Hospital-acquired pneumonia: Patients who have other health problems do not recover quickly from pneumonia, and in some cases it becomes life-threatening and may lead to death.

- Health care-acquired pneumonia: Most cases get well without taking antibiotics, but recovery happens faster when antibiotics are used. However, this type of pneumonia can develop to become severe in older adults.

- Aspiration pneumonia:  Severe cases may lead to long-term damage to the lungs.

Reports by the World Health Organization (WHO) show that, worldwide, pneumonia caused the death of 920,136 children, accounting for 16% of all deaths of children under 5 years old. Most cases reported by the WHO were in South Asia and sub-Saharan Africa.