Pneumococcal infection is an infection caused by the bacterium "Streptococcus pneumoniae". "S. pneumoniae" is a common member of the bacterial flora colonizing the nose and throat of 5–10% of healthy adults and 20–40% of healthy children. However, it is also the cause of significant disease being a leading cause of pneumonia, bacterial meningitis, and sepsis. The World Health Organization estimate that in 2005 pneumococcal infections were responsible for the death of 1.6 million children worldwide.

"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.

"Streptococcus pneumoniae" () is the most common bacterial cause of pneumonia in all age groups except newborn infants. "Streptococcus pneumoniae" is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia.

Other important Gram-positive causes of pneumonia are "Staphylococcus aureus" () and "Bacillus anthracis".

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.

Symptoms are similar to tuberculosis (TB), and include fever, fatigue, and weight loss. Pulmonary involvement is similar to TB, while diarrhea and abdominal pain are associated with gastrointestinal involvement.

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.

In humans, after an incubation period of 5–19 days, the symptoms of the disease range from inapparent illness to systemic illness with severe pneumonia. It presents chiefly as an atypical pneumonia. In the first week of psittacosis the symptoms mimic typhoid fever: prostrating high fevers, joint pains, diarrhea, conjunctivitis, nose bleeds and low level of white blood cells in the blood. Rose spots can appear and these are called Horder's spots. Spleen enlargement is common towards the end of the first week. It may become a serious lung infection. Diagnosis can be suspected in case of respiratory infection associated with splenomegaly and/or epistaxis. Headache can be so severe that it suggests meningitis and some nuchal rigidity is not unusual. Towards the end of the first week stupor or even coma can result in severe cases.

The second week is more akin to acute bacteremic pneumococcal pneumonia with continuous high fevers, headaches, cough, and dyspnea. X-rays show patchy infiltrates or a diffuse whiteout of lung fields.

Complications in the form of endocarditis, liver inflammation, inflammation of the heart's muscle, joint inflammation, keratoconjunctivitis (occasionally extranodal marginal zone lymphoma of the lacrimal gland/orbit), and neurologic complications (brain inflammation) may occasionally occur. Severe pneumonia requiring intensive-care support may also occur. Fatal cases have been reported (less than 1% of cases).

Usually the atypical causes also involve atypical symptoms:

- No response to common antibiotics such as sulfonamide and beta-lactams like penicillin.

- No signs and symptoms of lobar consolidation, meaning that the infection is restricted to small areas, rather than involving a whole lobe. As the disease progresses, however, the look can tend to lobar pneumonia.

- Absence of leukocytosis.

- Extrapulmonary symptoms, related to the causing organism.

- Moderate amount of sputum, or no sputum at all (i.e. non-productive).

- Lack of alveolar exudate.

- Despite general symptoms and problems with the upper respiratory tract (such as high fever, headache, a dry irritating cough followed later by a productive cough with radiographs showing consolidation), there are in general few physical signs. The patient looks better than the symptoms suggest.

"Mycobacterium avium-intracellulare" infection (MAI) is an atypical mycobacterial infection, i.e. one with nontuberculous mycobacteria or NTM, caused by "Mycobacterium avium" complex ("MAC"), which is made of three mycobacteria species, "M. avium", "M. intracellulare", and "M. chimaera". This infection causes respiratory illness in birds, pigs, and humans, especially in immunocompromised people. In the later stages of AIDS it can be very severe. It usually first presents as a persistent cough. It is typically treated with a series of three antibiotics for a period of at least six months.

"M. avium", "M. intracellulare", and "M. chimaera" are each saprotrophic organisms present in soil and water; entry into hosts is usually via the gastrointestinal tract, but also can be via the lungs.

MAC infections can cause fevers, diarrhea, malabsorption, as well as loss of appetite and weight loss, and can disseminate to the bone marrow. Therapy for MAI is typically resistant to standard mycobacterial therapies.

Pneumonia can cause respiratory failure by triggering acute respiratory distress syndrome (ARDS), which results from a combination of infection and inflammatory response. The lungs quickly fill with fluid and become stiff. This stiffness, combined with severe difficulties extracting oxygen due to the alveolar fluid, may require long periods of mechanical ventilation for survival.

Sepsis is a potential complication of pneumonia but occurs usually in people with poor immunity or hyposplenism. The organisms most commonly involved are "Streptococcus pneumoniae", "Haemophilus influenzae", and "Klebsiella pneumoniae". Other causes of the symptoms should be considered such as a myocardial infarction or a pulmonary embolism.

"C. psittaci" in birds is often systemic and infections can be inapparent, severe, acute or chronic with intermittent shedding. Signs in birds include "inflamed eyes, difficulty in breathing, watery droppings and green urates."

Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all.

This is occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by "Streptococcus pneumoniae", as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.

Infiltration commonly begins in the perihilar region (where the bronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe, but may affect any lobe or combination of lobes.

A hospital-acquired infection (HAI), also known as a nosocomial infection, is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is sometimes instead called a health care–associated infection (HAI or HCAI). Such an infection can be acquired in hospital, nursing home, rehabilitation facility, outpatient clinic, or other clinical settings. Infection is spread to the susceptible patient in the clinical setting by various means. Health care staff can spread infection, in addition to contaminated equipment, bed linens, or air droplets. The infection can originate from the outside environment, another infected patient, staff that may be infected, or in some cases, the source of the infection cannot be determined. In some cases the microorganism originates from the patient's own skin microbiota, becoming opportunistic after surgery or other procedures that compromise the protective skin barrier. Though the patient may have contracted the infection from their own skin, the infection is still considered nosocomial since it develops in the health care setting.

In the United States, the Centers for Disease Control and Prevention estimated roughly 1.7 million hospital-associated infections, from all types of microorganisms, including bacteria and fungi combined, cause or contribute to 99,000 deaths each year. In Europe, where hospital surveys have been conducted, the category of gram-negative infections are estimated to account for two-thirds of the 25,000 deaths each year. Nosocomial infections can cause severe pneumonia and infections of the urinary tract, bloodstream and other parts of the body. Many types are difficult to treat with antibiotics. In addition, antibiotic resistance can complicate treatment.

People with infectious pneumonia often have a productive cough, fever accompanied by shaking chills, shortness of breath, sharp or stabbing chest pain during deep breaths, and an increased rate of breathing. In the elderly, confusion may be the most prominent sign.

The typical signs and symptoms in children under five are fever, cough, and fast or difficult breathing. Fever is not very specific, as it occurs in many other common illnesses, may be absent in those with severe disease, malnutrition or in the elderly. In addition, a cough is frequently absent in children less than 2 months old. More severe signs and symptoms in children may include blue-tinged skin, unwillingness to drink, convulsions, ongoing vomiting, extremes of temperature, or a decreased level of consciousness.

Bacterial and viral cases of pneumonia usually present with similar symptoms. Some causes are associated with classic, but non-specific, clinical characteristics. Pneumonia caused by "Legionella" may occur with abdominal pain, diarrhea, or confusion, while pneumonia caused by "Streptococcus pneumoniae" is associated with rusty colored sputum, and pneumonia caused by "Klebsiella" may have bloody sputum often described as "currant jelly". Bloody sputum (known as hemoptysis) may also occur with tuberculosis, Gram-negative pneumonia, and lung abscesses as well as more commonly with acute bronchitis. "Mycoplasma" pneumonia may occur in association with swelling of the lymph nodes in the neck, joint pain, or a middle ear infection. Viral pneumonia presents more commonly with wheezing than does bacterial pneumonia. Pneumonia was historically divided into "typical" and "atypical" based on the belief that the presentation predicted the underlying cause. However, evidence has not supported this distinction, thus it is no longer emphasized.

Feline viral rhinotracheitis (FVR) is an upper respiratory or pulmonary infection of cats caused by "feline herpesvirus 1", of the family "Herpesviridae". It is also commonly referred to as feline influenza, feline coryza, and feline pneumonia but, as these terms describe other very distinct collections of respiratory symptoms, they are misnomers for the condition. Viral respiratory diseases in cats can be serious, especially in catteries and kennels. Causing one-half of the respiratory diseases in cats, FVR is the most important of these diseases and is found worldwide. The other important cause of feline respiratory disease is "feline calicivirus".

FVR is very contagious and can cause severe disease, including death from pneumonia in young kittens. It can cause flat-chested kitten syndrome, but most evidence for this is anecdotal. All members of the "Felidae" family are susceptible to FVR; in fact, FHV-1 has caused a fatal encephalitis in lions in Germany.

Additional problems may occur in the early stage of the illness. These may require specific treatment, and sometimes indicate severe illness or worse prognosis. The infection may trigger sepsis, a systemic inflammatory response syndrome of falling blood pressure, fast heart rate, high or abnormally low temperature, and rapid breathing. Very low blood pressure may occur at an early stage, especially but not exclusively in meningococcal meningitis; this may lead to insufficient blood supply to other organs. Disseminated intravascular coagulation, the excessive activation of blood clotting, may obstruct blood flow to organs and paradoxically increase the bleeding risk. Gangrene of limbs can occur in meningococcal disease. Severe meningococcal and pneumococcal infections may result in hemorrhaging of the adrenal glands, leading to Waterhouse-Friderichsen syndrome, which is often fatal.

The brain tissue may swell, pressure inside the skull may increase and the swollen brain may herniate through the skull base. This may be noticed by a decreasing level of consciousness, loss of the pupillary light reflex, and abnormal posturing. The inflammation of the brain tissue may also obstruct the normal flow of CSF around the brain (hydrocephalus). Seizures may occur for various reasons; in children, seizures are common in the early stages of meningitis (in 30% of cases) and do not necessarily indicate an underlying cause. Seizures may result from increased pressure and from areas of inflammation in the brain tissue. Focal seizures (seizures that involve one limb or part of the body), persistent seizures, late-onset seizures and those that are difficult to control with medication indicate a poorer long-term outcome.

Inflammation of the meninges may lead to abnormalities of the cranial nerves, a group of nerves arising from the brain stem that supply the head and neck area and which control, among other functions, eye movement, facial muscles, and hearing. Visual symptoms and hearing loss may persist after an episode of meningitis. Inflammation of the brain (encephalitis) or its blood vessels (cerebral vasculitis), as well as the formation of blood clots in the veins (cerebral venous thrombosis), may all lead to weakness, loss of sensation, or abnormal movement or function of the part of the body supplied by the affected area of the brain.

In adults, the most common symptom of meningitis is a severe headache, occurring in almost 90% of cases of bacterial meningitis, followed by nuchal rigidity (the inability to flex the neck forward passively due to increased neck muscle tone and stiffness). The classic triad of diagnostic signs consists of nuchal rigidity, sudden high fever, and altered mental status; however, all three features are present in only 44–46% of bacterial meningitis cases. If none of the three signs are present, acute meningitis is extremely unlikely. Other signs commonly associated with meningitis include photophobia (intolerance to bright light) and phonophobia (intolerance to loud noises). Small children often do not exhibit the aforementioned symptoms, and may only be irritable and look unwell. The fontanelle (the soft spot on the top of a baby's head) can bulge in infants aged up to 6 months. Other features that distinguish meningitis from less severe illnesses in young children are leg pain, cold extremities, and an abnormal skin color.

Nuchal rigidity occurs in 70% of bacterial meningitis in adults. Other signs include the presence of positive Kernig's sign or Brudziński sign. Kernig's sign is assessed with the person lying supine, with the hip and knee flexed to 90 degrees. In a person with a positive Kernig's sign, pain limits passive extension of the knee. A positive Brudzinski's sign occurs when flexion of the neck causes involuntary flexion of the knee and hip. Although Kernig's sign and Brudzinski's sign are both commonly used to screen for meningitis, the sensitivity of these tests is limited. They do, however, have very good specificity for meningitis: the signs rarely occur in other diseases. Another test, known as the "jolt accentuation maneuver" helps determine whether meningitis is present in those reporting fever and headache. A person is asked to rapidly rotate the head horizontally; if this does not make the headache worse, meningitis is unlikely.

Other problems can produce symptoms similar to those above, but from non-meningitic causes. This is called meningism or pseudomeningitis.

Meningitis caused by the bacterium "Neisseria meningitidis" (known as "meningococcal meningitis") can be differentiated from meningitis with other causes by a rapidly spreading petechial rash, which may precede other symptoms. The rash consists of numerous small, irregular purple or red spots ("petechiae") on the trunk, lower extremities, mucous membranes, conjuctiva, and (occasionally) the palms of the hands or soles of the feet. The rash is typically non-blanching; the redness does not disappear when pressed with a finger or a glass tumbler. Although this rash is not necessarily present in meningococcal meningitis, it is relatively specific for the disease; it does, however, occasionally occur in meningitis due to other bacteria. Other clues on the cause of meningitis may be the skin signs of hand, foot and mouth disease and genital herpes, both of which are associated with various forms of viral meningitis.

A subclinical infection (sometimes called a preinfection) is an infection that, being , is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals. An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

Occult pneumonia is a pneumonia that is not observable directly by the eye, but can only be shown indirectly, especially by radiography. Occult pneumonia can be made visible by chest X-rays.

The general symptoms "cough for more than 10 days" and "fever for more than 3 days" can indicate the presence of occult pneumonia, just as a temperature of 39 °C or higher and a high white blood cell count.

Administration of a pneumococcal vaccine decreases the incidence of occult pneumonia, which suggests that "Streptococcus pneumoniae" is a cause of occult pneumonia. Occult pneumonia, however, can also be the result of atypical pneumonia.

Although pneumococcal vaccination lowers the prevalence of occult pneumonia, it does not make radiographic diagnosis superfluous at patients with prolonged fever, cough or leukocytosis.

Etymology: the term is derived from the Latin "occultus" = hidden, secret and "pneumonia" = inflammation of the lungs > Greek: "pneuma" = wind and Indo-European: "pleumon" = floating, swimming.

Lower respiratory tract infection (LRTI), while often used as a synonym for pneumonia, can also be applied to other types of infection including lung abscess and acute bronchitis. Symptoms include shortness of breath, weakness, fever, coughing and fatigue.

There are a number of symptoms that are characteristic of lower respiratory tract infections. The two most common are bronchitis and edema. Influenza affects both the upper and lower respiratory tracts.

Antibiotics are the first line treatment for pneumonia; however, they are not effective or indicated for parasitic or viral infections. Acute bronchitis typically resolves on its own with time.

In 2015 there were about 291 million cases. These resulted in 2.74 million deaths down from 3.4 million deaths in 1990. This was 4.8% of all deaths in 2013.

Bronchitis describes the swelling or inflammation of the bronchial tubes. Additionally, bronchitis is described as either acute or chronic depending on its presentation and is also further described by the causative agent. Acute bronchitis can be defined as acute bacterial or viral infection of the larger airways in healthy patients with no history of recurrent disease. It affects over 40 adults per 1000 each year and consists of transient inflammation of the major bronchi and trachea. Most often it is caused by viral infection and hence antibiotic therapy is not indicated in immunocompetent individuals. Viral bronchitis can sometimes be treated using antiviral medications depending on the virus causing the infection, and medications such as anti-inflammatory drugs and expectorants can help mitigate the symptoms. Treatment of acute bronchitis with antibiotics is common but controversial as their use has only moderate benefit weighted against potential side effects (nausea and vomiting), increased resistance, and cost of treatment in a self-limiting condition. Beta2 agonists are sometimes used to relieve the cough associated with acute bronchitis. In a recent systematic review it was found there was no evidence to support their use.

Initial signs of FVR include coughing, sneezing, nasal discharge, conjunctivitis, and sometimes fever (up to 106) and loss of appetite. These usually resolve within four to seven days, but secondary bacterial infections can cause the persistence of clinical signs for weeks. Frontal sinusitis and empyema can also result.

FHV-1 also has a predilection for corneal epithelium, resulting in corneal ulcers, often pinpoint or dendritic in shape. Other ocular signs of FHV-1 infection include conjunctivitis, keratitis, keratoconjunctivitis sicca (decreased tear production), and corneal sequestra. Infection of the nasolacrimal duct can result in chronic epiphora (excess tearing). Ulcerative skin disease can also result from FHV-1 infection. FHV-1 can also cause abortion in pregnant queens, usually at the sixth week of gestation, although this may be due to systemic effects of the infection rather than the virus directly.

In chronic nasal and sinus disease of cats, FHV-1 may play more of an initiating role than an ongoing cause. Infection at an early age may permanently damage nasal and sinus tissue, causing a disruption of ciliary clearance of mucus and bacteria, and predispose these cats to chronic bacterial infections.

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

Indwelling catheters have recently been identified with hospital acquired infections. Procedures using Intravascular Antimicrobial Lock Therapy can reduce infections that are unexposed to blood-borne antibiotics. Introducing antibiotics, including ethanol, into the catheter (without flushing it into the bloodstream) reduces the formation of biofilms.

Contact transmission is divided into two subgroups: direct-contact transmission and indirect-contact transmission.

The clinical presentation of both the adult and pediatric patient with pleural empyema depends upon several factors, including the causative micro-organism. Most cases present themselves in the setting of a pneumonia, although up to one third of patients do not have clinical signs of pneumonia and as many as 25% of cases are associated with trauma (including surgery). Typical symptoms include cough, chest pain, shortness of breath and fever.

Bovine viral diarrhea (BVD) or bovine viral diarrhoea (UK English), and previously referred to as bovine virus diarrhoea (BVD), is a significant economic disease of cattle that is endemic in the majority of countries throughout the world. The causative agent, bovine viral diarrhea virus (BVDV), is a member of the "Pestivirus" genus of the family Flaviviridae.

BVD infection results in a wide variety of clinical signs, due to its immunosuppressive effects, as well as having a direct effect on respiratory disease and fertility. In addition, BVD infection of a susceptible dam during a certain period of gestation can result in the production of a persistently infected (PI) fetus.

PI animals recognise intra-cellular BVD viral particles as ‘self’ and shed virus in large quantities throughout life; they represent the cornerstone of the success of BVD as a disease.