The bacterium Streptococcus<unk> e, a common cause of<unk> , imaged by an electron microscope<unk>  is due to infections caused primarily by bacteria or viruses and less commonly by fungi and parasites. Although more than 100 strains of infectious agents have been identified, only a few are responsible for the majority of cases. Mixed infections with both viruses and bacteria may occur in roughly 45% of infections in children and 15% of infections in adults. A causative agent may not be isolated in about half of cases despite careful testing. In an active population-based surveillance for community-acquired<unk>  requiring hospitalization in five hospitals in Chicago and Nashville from January 2010 through June 2012, 2259 patients were identified who had radiographic evidence of<unk>  and specimens that could be tested for the responsible pathogen. Most patients (62%) had no detectable pathogens in their sample, and unexpectedly, respiratory viruses were detected more frequently than bacteria. Specifically, 23% had<unk>  or more viruses, 11% had<unk>  or more bacteria, 3% had both bacterial and viral pathogens, and 1% had a fungal or mycobacterial infection. "The most common pathogens were human rhinovirus (in 9% of patients), influenza virus (in 6%), and Streptococcus<unk> e (in 5%)." The term<unk>  is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as<unk> is. Factors that predispose to<unk>  include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, sickle cell disease (SCD), asthma, chronic kidney disease, liver disease, and biological aging. Additional risks in children include not being breastfed, exposure to cigarette smoke and other air pollution, malnutrition, and poverty. The use of acid-suppressing medications – such as proton-pump inhibitors or H2 blockers – is associated with an increased risk of<unk> . Approximately 10% of people who require mechanical ventilation develop ventilator-associated<unk> , and people with a gastric feeding tube have an increased risk of developing aspiration<unk> . For people with certain variants of the FER gene, the risk of death is reduced in sepsis caused by<unk> . However, for those with TLR6 variants, the risk of getting Legionnaires' disease is increased. Bacteria Cavitating<unk>  due to MRSA as seen on a CT scan Bacteria are the most common cause of community-acquired<unk>  (CAP), with Streptococcus<unk> e isolated in nearly 50% of cases. Other commonly isolated bacteria include Haemophilus influenzae in 20%, Chlamydophila<unk> e in 13%, and Mycoplasma<unk> e in 3% of cases; Staphylococcus aureus; Moraxella catarrhalis; and Legionella<unk> eumophila. A number of drug-resistant versions of the above infections are becoming more common, including drug-resistant Streptococcus<unk> e (DRSP) and methicillin-resistant Staphylococcus aureus (MRSA). The spreading of organisms is facilitated by certain risk factors. Alcoholism is associated with Streptococcus<unk> e, anaerobic organisms, and Mycobacterium tuberculosis; smoking facilitates the effects of Streptococcus<unk> e, Haemophilus influenzae, Moraxella catarrhalis, and Legionella<unk> eumophila. Exposure to birds is associated with Chlamydia psittaci; farm animals with Coxiella burnetti; aspiration of stomach contents with anaerobic organisms; and cystic fibrosis with Pseudomonas aeruginosa and Staphylococcus aureus. Streptococcus<unk> e is more common in the winter, and it should be suspected in persons aspirating a large number of anaerobic organisms. Viruses SARS In adults, viruses account for about<unk>  third of<unk>  cases, and in children for about 15% of them. Commonly implicated agents include rhinoviruses, coronaviruses, influenza virus, respiratory syncytial virus (RSV), adenovirus, and parainfluenza. Herpes simplex virus rarely causes<unk> , except in groups such as newborns, persons with cancer, transplant recipients, and people with significant burns. After organ transplantation or in otherwise immunocompromised persons, there are high rates of cytomegalovirus<unk> . Those with viral infections may be secondarily infected with the bacteria Streptococcus<unk> e, Staphylococcus aureus, or Haemophilus influenzae, particularly when other health problems are present. Different viruses predominate at different times of the year; during flu season, for example, influenza may account for more than half of all viral cases. Outbreaks of other viruses also occur occasionally, including hantaviruses and coronaviruses. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can also result in<unk> . Fungi Fungal<unk>  is uncommon, but occurs more commonly in individuals with weakened immune systems due to AIDS, immunosuppressive drugs, or other medical problems. It is most often caused by Histoplasma capsulatum, Blastomyces, Cryptococcus neoformans,<unk> eumocystis jiroveci <unk> eumocystis<unk> , or PCP), and Coccidioides immitis. Histoplasmosis is most common in the Mississippi River basin, and coccidioidomycosis is most common in the Southwestern United States. The number of cases of fungal<unk>  has been increasing in the latter half of the 20th century due to increasing travel and rates of immunosuppression in the population. For people infected with HIV/AIDS, PCP is a common opportunistic infection. Parasites A variety of parasites can affect the lungs, including Toxoplasma gondii, Strongyloides stercoralis, Ascaris lumbricoides, and Plasmodium malariae. These organisms typically enter the body through direct contact with the skin, ingestion, or via an insect vector. Except for Paragonimus westermani, most parasites do not specifically affect the lungs but involve the lungs secondarily to other sites. Some parasites, in particular those belonging to the Ascaris and Strongyloides genera, stimulate a strong eosinophilic reaction, which may result in eosinophilic<unk> . In other infections, such as malaria, lung involvement is due primarily to cytokine-induced systemic inflammation. In the developed world, these infections are most common in people returning from travel or in immigrants. Around the world, parasitic<unk>  is most common in the immunodeficient. Noninfectious Idiopathic interstitial<unk>  or noninfectious<unk>  is a class of diffuse lung diseases. They include diffuse alveolar damage, organizing<unk> , nonspecific interstitial<unk> , lymphocytic interstitial<unk> , desquamative interstitial<unk> , respiratory bronchiolitis interstitial lung disease, and usual interstitial<unk> . Lipoid<unk>  is another rare cause due to lipids entering the lung. These lipids can either be inhaled or spread to the lungs from elsewhere in the body.
Pneumonia
Chronic obstructive pulmonary disease
Aspiration pneumonia
Pulmonary embolism
Bacterial pneumonia
Pulmonary edema
Dust pneumonia
Asthma