CAP is common worldwide, and a major cause of death in all age groups. In children, most deaths (over two million a year) occur in newborn period. According to a World Health Organization estimate, one in three newborn deaths are from pneumonia. Mortality decreases with age until late adulthood, with the elderly at risk for CAP and its associated mortality.

More CAP cases occur during the winter than at other times of the year. CAP is more common in males than females, and more common in black people than Caucasians. Patients with underlying illnesses (such as Alzheimer's disease, cystic fibrosis, COPD, tobacco smoking, alcoholism or immune-system problems) have an increased risk of developing pneumonia.

Several studies found that healthcare-associated pneumonia is the second most common type of pneumonia, occurring less commonly than community-acquired pneumonia but more frequently than hospital-acquired pneumonia and ventilator-associated pneumonia. In a recent observational study, the rates for CAP, HCAP and HAP were 60%, 25% and 15% respectively. Patients with HCAP are older and more commonly have simultaneous health problems (such as previous stroke, heart failure and diabetes).

The number of residents in long term care facilities is expected to rise dramatically over the next 30 years. These older adults are known to develop pneumonia 10 times more than their community-dwelling peers, and hospital admittance rates are 30 times higher.

CAP may be prevented by treating underlying illnesses increasing its risk, by smoking cessation and vaccination of children and adults. Vaccination against "haemophilus influenzae" and "streptococcus pneumoniae" in the first year of life has reduced their role in childhood CAP. A vaccine against "streptococcus pneumoniae", available for adults, is recommended for healthy individuals over 65 and all adults with COPD, heart failure, diabetes mellitus, cirrhosis, alcoholism, cerebrospinal fluid leaks or who have had a splenectomy. Re-vaccination may be required after five or ten years.

Patients who are vaccinated against "streptococcus pneumoniae", health professionals, nursing-home residents and pregnant women should be vaccinated annually against influenza. During an outbreak, drugs such as amantadine, rimantadine, zanamivir and oseltamivir have been demonstrated to prevent influenza.

Nursing home-acquired pneumonia is an important subgroup of HCAP. Residents of long term care facilities may become infected through their contacts with the healthcare system; as such, the microbes responsible for their pneumonias may be different from those traditionally seen in community-dwelling patients, requiring therapy with different antibiotics. Other groups include patients who are admitted as a day case for regular hemodialysis or intravenous infusion (for example, chemotherapy). Especially in the very old and in demented patients, HCAP is likely to present with atypical symptoms.

Smoking cessation and reducing indoor air pollution, such as that from cooking indoors with wood or dung, are both recommended. Smoking appears to be the single biggest risk factor for pneumococcal pneumonia in otherwise-healthy adults. Hand hygiene and coughing into one's sleeve may also be effective preventative measures. Wearing surgical masks by the sick may also prevent illness.

Appropriately treating underlying illnesses (such as HIV/AIDS, diabetes mellitus, and malnutrition) can decrease the risk of pneumonia. In children less than 6 months of age, exclusive breast feeding reduces both the risk and severity of disease. In those with HIV/AIDS and a CD4 count of less than 200 cells/uL the antibiotic trimethoprim/sulfamethoxazole decreases the risk of "Pneumocystis pneumonia" and is also useful for prevention in those that are immunocomprised but do not have HIV.

Testing pregnant women for Group B Streptococcus and "Chlamydia trachomatis", and administering antibiotic treatment, if needed, reduces rates of pneumonia in infants; preventive measures for HIV transmission from mother to child may also be efficient. Suctioning the mouth and throat of infants with meconium-stained amniotic fluid has not been found to reduce the rate of aspiration pneumonia and may cause potential harm, thus this practice is not recommended in the majority of situations. In the frail elderly good oral health care may lower the risk of aspiration pneumonia. Zinc supplementation in children 2 months to five years old appears to reduce rates of pneumonia.

When influenza outbreaks occur, medications such as amantadine or rimantadine may help prevent the condition; however are associated with side effects. Zanamivir or oseltamivir decrease the chance that those exposed will develop symptoms; however, it is recommended that potential side effects are taken into account.

Vaccination helps prevent bronchopneumonia, mostly against influenza viruses, adenoviruses, measles, rubella, streptococcus pneumoniae, haemophilus influenzae, diphtheria, bacillus anthracis, chickenpox, and bordetella pertussis.

Gram-negative bacteria are seen less frequently: "Haemophilus influenzae" (), "Klebsiella pneumoniae" (), "Escherichia coli" (), "Pseudomonas aeruginosa" (), "Bordetella pertussis", and "Moraxella catarrhalis" are the most common.

These bacteria often live in the gut and enter the lungs when contents of the gut (such as vomit or faeces) are inhaled.

Lower respiratory infectious disease is the fifth-leading cause of death and the combined leading infectious cause of death, being responsible for 2·74 million deaths worldwide. This is generally similar to estimates in the 2010 Global Burden of Disease study.

This total only accounts for "Streptococcus pneumoniae" and "Haemophilus Influenzae" infections and does not account for atypical or nosocomial causes of lower respiratory disease, therefore underestimating total disease burden.

"Klebsiella" resistant strains have been recorded in USA with a roughly threefold increase in Chicago cases, quarantined individuals in Israel, United Kingdom and parts of Europe, possible ground zero, or location of emergence, is the India-Pakistan border.

A strain known as Carbapenem-Resistant Klebsiella pneumonia (CRKP) was estimated to be involved in 350 cases in Los Angeles county between June and December 2010.

When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial cause and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS)

and measles.

The most common causative organisms are (often intracellular living) bacteria:

- "Chlamydophila pneumoniae": Mild form of pneumonia with relatively mild symptoms.

- "Chlamydophila psittaci": Causes psittacosis.

- "Coxiella burnetii": Causes Q fever.

- "Francisella tularensis": Causes tularemia.

- "Legionella pneumophila": Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires' disease.

- "Mycoplasma pneumoniae": Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Atypical pneumonia can also have a fungal, protozoan or viral cause.In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.

Atypical bacteria causing pneumonia are "Coxiella burnetii", "Chlamydophila pneumoniae" (), "Mycoplasma pneumoniae" (), and "Legionella pneumophila".

The term "atypical" does not relate to how commonly these organisms cause pneumonia, how well it responds to common antibiotics or how typical the symptoms are; it refers instead to the fact that these organisms have atypical or absent cell wall structures and do not take up Gram stain in the same manner as gram-negative and gram-positive organisms.

Pneumonia caused by "Yersinia pestis" is usually called pneumonic plague.

Most strains of "H. influenzae" are opportunistic pathogens; that is, they usually live in their host without causing disease, but cause problems only when other factors (such as a viral infection, reduced immune function or chronically inflamed tissues, e.g. from allergies) create an opportunity. They infect the host by sticking to the host cell using trimeric autotransporter adhesins.

Naturally acquired disease caused by "H. influenzae" seems to occur in humans only. In infants and young children, "H. influenzae" type b (Hib) causes bacteremia, pneumonia, epiglottitis and acute bacterial meningitis. On occasion, it causes cellulitis, osteomyelitis, and infectious arthritis. It is one cause of neonatal infection.

Due to routine use of the Hib conjugate vaccine in the U.S. since 1990, the incidence of invasive Hib disease has decreased to 1.3/100,000 in children. However, Hib remains a major cause of lower respiratory tract infections in infants and children in developing countries where the vaccine is not widely used. Unencapsulated "H. influenzae" strains are unaffected by the Hib vaccine and cause ear infections (otitis media), eye infections (conjunctivitis), and sinusitis in children, and are associated with pneumonia.

In terms of the pathophysiology of Klebsiella pneumonia we see neutrophil myeloperoxidase defense against "K P".Oxidative inactivation of elastase is involved, while LBP helps transfer bacteria cell wall elements to the cells.

"S. pneumoniae" is normally found in the nose and throat of 5–10% of healthy adults and 20–40% of healthy children. It can be found in higher amounts in certain environments, especially those where people are spending a great deal of time in close proximity to each other (day-care centers, military barracks). It attaches to nasopharyngeal cells through interaction of bacterial surface adhesins. This normal colonization can become infectious if the organisms are carried into areas such as the Eustachian tube or nasal sinuses where it can cause otitis media and sinusitis, respectively. Pneumonia occurs if the organisms are inhaled into the lungs and not cleared (again, viral infection, or smoking-induced ciliary paralysis might be contributing factors). The organism's polysaccharide capsule makes it resistant to phagocytosis and if there is no pre-existing anticapsular antibody alveolar macrophages cannot adequately kill the pneumococci. The organism spreads to the blood stream (where it can cause bacteremia) and is carried to the meninges, joint spaces, bones, and peritoneal cavity, and may result in meningitis, brain abscess, septic arthritis, or osteomyelitis.

"S. pneumoniae" has several virulence factors, including the polysaccharide capsule mentioned earlier, that help it evade a host's immune system. It has pneumococcal surface proteins that inhibit complement-mediated opsonization, and it secretes IgA1 protease that will destroy secretory IgA produced by the body and mediates its attachment to respiratory mucosa.

The risk of pneumococcal infection is much increased in persons with impaired IgG synthesis, impaired phagocytosis, or defective clearance of pneumococci. In particular, the absence of a functional spleen, through congenital asplenia, surgical removal of the spleen, or sickle-cell disease predisposes one to a more severe course of infection (overwhelming post-splenectomy infection) and prevention measures are indicated (see asplenia).

People with a compromised immune system, such as those living with HIV, are also at higher risk of pneumococcal disease. In HIV patients with access to treatment, the risk of invasive pneumoccal disease is 0.2–1% per year and has a fatality rate of 8%.

There is an association between pneumococcal pneumonia and influenza. Damage to the lining of the airways (respiratory epithelium) and upper respiratory system caused by influenza may facilitate pneumococcal entry and infection.

Other risk factors include smoking, injection drug use, Hepatitis C, and COPD.

"S. pneumoniae" is responsible for 15–50% of all episodes of community acquired pneumonia, 30–50% of all cases of acute otitis media, and a significant proportion of bloodstream infections and bacterial meningitis.

As estimated by WHO in 2005 it killed about 1.6 million children every year worldwide with 0.7–1 million of them being under the age of five. The majority of these deaths were in developing countries.

In 1930, two major categories of "H. influenzae" were defined: the unencapsulated strains and the encapsulated strains. Encapsulated strains were classified on the basis of their distinct capsular antigens. There are six generally recognized types of encapsulated "H. influenzae": a, b, c, d, e, and f.

Genetic diversity among unencapsulated strains is greater than within the encapsulated group. Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes; however, they can be classified by multilocus sequence typing. The pathogenesis of "H. influenzae" infections is not completely understood, although the presence of the capsule in encapsulated type b (Hib), a serotype causing conditions such as epiglottitis, is known to be a major factor in virulence. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the nonimmune host. The unencapsulated strains are almost always less invasive; they can, however, produce an inflammatory response in humans, which can lead to many symptoms. Vaccination with Hib conjugate vaccine is effective in preventing Hib infection, but does not prevent infection with NTHi strains.

Pneumococcal pneumonia is a type of bacterial pneumonia that is specifically caused by Streptococcus pneumoniae. "S. pneumoniae" is also called pneumococcus. It is the most common bacterial pneumonia found in adults. The estimated number of Americans with pneumococcal pneumonia is 900,000 annually, with almost 400,000 cases hospitalized and fatalities accounting for 5-7% of these cases.

The symptoms of pneumococcal pneumonia can occur suddenly, typically presenting as a severe chill, later including a severe fever, cough, shortness of breath, rapid breathing, and chest pains. Other symptoms like nausea, vomiting, headache, fatigue, and muscle aches could also accompany the original symptoms. Sometimes the coughing can produce rusty or blood-streaked sputum. In 25% of cases, a parapneumonic effusion may occur. Chest X-rays will typically show lobar consolidation or patchy infiltrates.

In most cases, once pneumococcal pneumonia has been identified, doctors will prescribe antibiotics. These antibiotic usually help alleviate and eliminate symptoms between 12 and 36 hours after being taken. Despite most antibiotics' effectiveness in treating the disease, sometimes the bacteria can resist the antibiotics, causing symptoms to worsen. Additionally, age and health of the infected patient can contribute to the effectiveness of the antibiotics. A vaccine has also been developed for the prevention of pneumococcal pneumonia, recommended to children under age five as well as adults over the age of 65.

While it has been commonly known that the influenza virus increases one's chances of contracting pneumonia or meningitis caused by the streptococcus pneumonaie bacteria, new medical research in mice indicates that the flu is actually a necessary component for the transmission of the disease. Researcher Dimitri Diavatopoulo from the Radboud University Nijmegen Medical Centre in the Netherlands describes his observations in mice, stating that in these animals, the spread of the bacteria only occurs between animals already infected with the influenza virus, not between those without it. He says that these findings have only been inclusive in mice, however, he believes that the same could be true for humans.

Viral pneumonia occurs in about 200 million people a year which includes about 100 million children and 100 million adults.

While antibiotics with activity specifically against "M. pneumoniae" are often used (e.g., erythromycin, doxycycline), it is unclear if these result in greater benefit than using antibiotics without specific activity against this organism in those with an infection acquired in the community.

Common causes of viral pneumonia are:

- "Influenza virus" A and B

- "Respiratory syncytial virus" (RSV)

- "Human parainfluenza viruses" (in children)

Rarer viruses that commonly result in pneumonia include:

- "Adenoviruses" (in military recruits)

- "Metapneumovirus"

- "Severe acute respiratory syndrome virus" (SARS coronavirus)

- "Middle East respiratory syndrome virus" (MERS coronavirus)

Viruses that primarily cause other diseases, but sometimes cause pneumonia include:

- "Herpes simplex virus" (HSV), mainly in newborns or young children

- "Varicella-zoster virus" (VZV)

- "Measles virus"

- "Rubella virus"

- "Cytomegalovirus" (CMV), mainly in people with immune system problems

- "Smallpox virus"

- "dengue virus"

The most commonly identified agents in children are "respiratory syncytial virus", "rhinovirus", "human metapneumovirus", "human bocavirus", and "parainfluenza viruses".

"Mycoplasma pneumoniae" is spread through respiratory droplet transmission. Once attached to the mucosa of a host organism, "M. pneumoniae" extracts nutrients, grows, and reproduces by binary fission. Attachment sites include the upper and lower respiratory tract, causing pharyngitis, bronchitis, and pneumonia. The infection caused by this bacterium is called atypical pneumonia because of its protracted course and lack of sputum production and wealth of extrapulmonary symptoms. Chronic "Mycoplasma" infections have been implicated in the pathogenesis of rheumatoid arthritis and other rheumatological diseases.

"Mycoplasma" atypical pneumonia can be complicated by Stevens–Johnson syndrome, autoimmune hemolytic anemia, cardiovascular diseases, encephalitis, or Guillain–Barré syndrome.

While the "Haemophilus influenzae" bacteria is unable to survive in any environment outside of the human body, humans can carry the bacteria within their bodies without developing any symptoms of the disease. It spreads through the air when an individual carrying the bacteria coughs or sneezes. The risk of developing "Haemophilus" meningitis is most directly related to an individual's vaccination history, as well as the vaccination history of the general public. Herd immunity, or the protection that unvaccinated individuals experience when the majority of others in their proximity are vaccinated, does help in the reduction of meningitis cases, but it does not guarantee protection from the disease. Contact with other individuals with the disease also vastly increases the risk of infection. A child in the presence of family members sick with "Haemophilus" meningitis or carrying the bacteria is 585 times more likely to catch "Haemophilus" meningitis. Additionally, siblings of individuals with the Haemophilus influenzae meningitis receive reduced benefits from certain types of immunization. Similarly, children under two years of age have a greater risk of contracting the disease when attending day care, especially in their first month of attendance, due to the maintained contact with other children who might be asymptomatic carriers of the Hib bacteria.

Some patients may develop pneumonia, lymphadenopathy, or septic arthritis.