The pleural space can be invaded by fluid, air, and particles from different parts of the body which fairly complicates the diagnosis. Viral infection (coxsackie B virus, HRSV, CMV, adenovirus, EBV, parainfluenza, influenza) is the most common cause of pleurisy. However, many other different conditions can cause pleuritic chest pain:

- Aortic dissections

- Autoimmune disorders such as systemic lupus erythematosus (or drug-induced lupus erythematosus), Autoimmune hepatitis (AIH), rheumatoid arthritis and Behçet's disease.

- Bacterial infections associated with pneumonia and tuberculosis

- Chest injuries (blunt or penetrating)

- Familial Mediterranean fever, an inherited condition that often causes fever and swelling in the abdomen or the lungs

- Fungal or parasitic infections

- Heart surgery, especially coronary-artery bypass grafting

- Cardiac problems (ischemia, pericarditis)

- Inflammatory bowel disease

- Lung cancer and lymphoma

- Other lung diseases like cystic fibrosis, sarcoidosis, asbestosis, lymphangioleiomyomatosis, and mesothelioma

- Pneumothorax

- Pulmonary embolisms, which are blood clots that enter the lungs

When the space between the pleurae starts to fill with fluid, as in pleural effusion, the chest pain can be eased but a shortness of breath can result, since the lungs need room to expand during breathing. Some cases of pleuritic chest pain are idiopathic, which means that the exact cause cannot be determined.

Pleurisy and other disorders of the pleurae can be serious, depending on what caused them. Generally, pleurisy treatment has an excellent prognosis, but if left untreated it can cause severe complications. For example, a resulting pulmonary heart disease cor pulmonale, which manifests itself with an inflammation of the arms and legs, can lead to heart failure. If the conditions that caused the pleurisy or other pleural disorders were adequately diagnosed and treated early, one can expect a full recovery. Help of a pulmonologist (respiratory physician in the U.K. and Australia) may be enlisted to address the underlying cause and chart post-illness rehabilitation.

When a pleural effusion has been determined to be exudative, additional evaluation is needed to determine its cause, and amylase, glucose, pH and cell counts should be measured.

- Red blood cell counts are elevated in cases of bloody effusions (for example after heart surgery or hemothorax from incomplete evacuation of blood).

- Amylase levels are elevated in cases of esophageal rupture, pancreatic pleural effusion, or cancer.

- Glucose is decreased with cancer, bacterial infections, or rheumatoid pleuritis.

- pH is low in empyema (<7.2) and may be low in cancer.

- If cancer is suspected, the pleural fluid is sent for cytology. If cytology is negative, and cancer is still suspected, either a thoracoscopy, or needle biopsy of the pleura may be performed.

- Gram staining and culture should also be done.

- If tuberculosis is possible, examination for "Mycobacterium tuberculosis" (either a Ziehl–Neelsen or Kinyoun stain, and mycobacterial cultures) should be done. A polymerase chain reaction for tuberculous DNA may be done, or adenosine deaminase or interferon gamma levels may also be checked.

The most common causes of exudative pleural effusions are bacterial pneumonia, cancer (with lung cancer, breast cancer, and lymphoma causing approximately 75% of all malignant pleural effusions), viral infection, and pulmonary embolism.

Another common cause is after heart surgery, when incompletely drained blood can lead to an inflammatory response that causes exudative pleural fluid.

Conditions associated with exudative pleural effusions:

- Parapneumonic effusion due to pneumonia

- Malignancy (either lung cancer or metastases to the pleura from elsewhere)

- Infection (empyema due to bacterial pneumonia)

- Trauma

- Pulmonary infarction

- Pulmonary embolism

- Autoimmune disorders

- Pancreatitis

- Ruptured esophagus (Boerhaave's syndrome)

- Rheumatoid pleurisy

- Drug-induced lupus

According to a recent study, the main risk factors for RA-ILD are advancing age, male sex, greater RA disease activity, rheumatoid factor (RF) positivity, and elevated titers of anticitrullinated protein antibodies such as anticyclic citrullinated peptide. Cigarette smoking also appears to increase risk of RA-ILD, especially in patients with human leukocyte antigen DRB1.

A recently published retrospective study by a team from Beijing Chao-Yang Hospital in Beijing, China, supported three of the risk factors listed for RA-ILD and identified an additional risk factor. In that study of 550 RA patients, logistic regression analysis of data collected on the 237 (43%) with ILD revealed that age, smoking, RF positivity, and elevated lactate dehydrogenase closely correlated with ILD.

Recent studies have identified risk factors for disease progression and mortality. A retrospective study of 167 patients with RA-ILD determined that the usual interstitial pneumonia (UIP) pattern on high-resolution computed tomography (HRCT) was a risk factor for progression, as were severe disease upon diagnosis and rate of change in pulmonary function test results in the first 6 months after diagnosis.

A study of 59 RA-ILD patients found no median survival difference between those with the UIP pattern and those without it. But the UIP group had more deaths, hospital admissions, need for supplemental oxygen, and decline in lung function.

Other causes of pleural effusion include tuberculosis (though stains of pleural fluid are only rarely positive for acid-fast bacilli, this is the most common cause of pleural effusions in some developing countries), autoimmune disease such as systemic lupus erythematosus, bleeding (often due to chest trauma), chylothorax (most commonly caused by trauma), and accidental infusion of fluids.

Less common causes include esophageal rupture or pancreatic disease, intra-abdominal abscesses, rheumatoid arthritis, asbestos pleural effusion, mesothelioma, Meigs' syndrome (ascites and pleural effusion due to a benign ovarian tumor), and ovarian hyperstimulation syndrome.

Pleural effusions may also occur through medical or surgical interventions, including the use of medications (pleural fluid is usually eosinophilic), coronary artery bypass surgery, abdominal surgery, endoscopic variceal sclerotherapy, radiation therapy, liver or lung transplantation, and intra- or extravascular insertion of central lines.

The exact cause of rheumatoid lung disease is unknown. However, associated factors could be due largely to smoking. Sometimes, the medicines used to treat rheumatoid arthritis, especially methotrexate, may result in lung disease.

Prevention's:

- Stop smoking: Chemicals found in cigarettes can irritate already delicate lung tissue, leading to further complications.

- Having regular checkups: The doctor could listen to lungs and monitor breathing, because lung problems that are detected early can be easier to treat.

Pleuropneumonia is inflammation of the lungs and pleura, pleurisy being the inflammation of the pleura alone.

A number of factors make people more susceptible to TB infections. The most important risk factor globally is HIV; 13% of all people with TB are infected by the virus. This is a particular problem in sub-Saharan Africa, where rates of HIV are high. Of people without HIV who are infected with tuberculosis, about 5–10% develop active disease during their lifetimes; in contrast, 30% of those coinfected with HIV develop the active disease.

Tuberculosis is closely linked to both overcrowding and malnutrition, making it one of the principal diseases of poverty. Those at high risk thus include: people who inject illicit drugs, inhabitants and employees of locales where vulnerable people gather (e.g. prisons and homeless shelters), medically underprivileged and resource-poor communities, high-risk ethnic minorities, children in close contact with high-risk category patients, and health-care providers serving these patients.

Chronic lung disease is another significant risk factor. Silicosis increases the risk about 30-fold. Those who smoke cigarettes have nearly twice the risk of TB compared to nonsmokers.

Other disease states can also increase the risk of developing tuberculosis. These include alcoholism and diabetes mellitus (three-fold increase).

Certain medications, such as corticosteroids and infliximab (an anti-αTNF monoclonal antibody), are becoming increasingly important risk factors, especially in the developed world.

Genetic susceptibility also exists, for which the overall importance remains undefined.

Progression from TB infection to overt TB disease occurs when the bacilli overcome the immune system defenses and begin to multiply. In primary TB disease (some 1–5% of cases), this occurs soon after the initial infection. However, in the majority of cases, a latent infection occurs with no obvious symptoms. These dormant bacilli produce active tuberculosis in 5–10% of these latent cases, often many years after infection.

The risk of reactivation increases with immunosuppression, such as that caused by infection with HIV. In people coinfected with "M. tuberculosis" and HIV, the risk of reactivation increases to 10% per year. Studies using DNA fingerprinting of "M. tuberculosis" strains have shown reinfection contributes more substantially to recurrent TB than previously thought, with estimates that it might account for more than 50% of reactivated cases in areas where TB is common. The chance of death from a case of tuberculosis is about 4% as of 2008, down from 8% in 1995.

The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects.

Hypersensitivity pneumonitis may also be called many different names, based on the provoking antigen. These include:

Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4–7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk."

Most cases of COVID-19 are not severe enough to require mechanical ventilation or alternatives, but a percentage of cases are. The type of respiratory support for individuals with COVID-19 related respiratory failure is being actively studied for people in hospital, with some evidence that intubation can be avoided with a high flow nasal cannula or bi-level positive airway pressure. Whether either of these two leads to the same benefit for people who are critically ill is not known. Some doctors prefer staying with invasive mechanical ventilation when available because this technique limits the spread of aerosol particles compared to a high flow nasal cannula.

Severe cases are most common in older adults (those older than 60 years, and especially those older than 80 years). Many developed countries do not have enough hospital beds per capita, which limits a health system's capacity to handle a sudden spike in the number of COVID-19 cases severe enough to require hospitalisation. This limited capacity is a significant driver behind calls to "flatten the curve"—to lower the speed at which new cases occur and thus keep the number of persons sick at any one time lower. One study in China found 5% were admitted to intensive care units, 2.3% needed mechanical support of ventilation, and 1.4% died. In China, approximately 30% of people in hospital with COVID-19 are eventually admitted to ICU.

The impact of the pandemic and its mortality rate are different for men and women. Mortality is higher in men in studies conducted in China and Italy. The highest risk for men is in their 50s, with the gap between men and women closing only at 90. In China, the death rate was 2.8 percent for men and 1.7 percent for women. The exact reasons for this sex-difference is not known, but genetic and behavioural factors could be a reason. Sex-based immunological differences, lesser prevalence of smoking in women and men developing co-morbid conditions such as hypertension at a younger age than women could have contributed to the higher mortality in men. In Europe, 57% of the infected individuals were men and 72% of those died with COVID-19 were men. As of April 2020, the US government is not tracking sex-related data of COVID-19 infections. Research has shown that viral illnesses like Ebola, HIV, influenza and SARS affect men and women differently. A higher percentage of health workers, particularly nurses, are women, and they have a higher chance of being exposed to the virus. School closures, lockdowns and reduced access to healthcare following the 2019–20 coronavirus pandemic may deferentially affect the genders and possibly exaggerate existing gender disparity.

Chronic kidney disease, for example, is sometimes a sequela of diabetes, "chronic constipation" or more accurately "obstipation" (that is, difficulty in passing stool) is a sequela to an intestinal obstruction, and neck pain is a common sequela of whiplash or other trauma to the cervical vertebrae. Post-traumatic stress disorder may be a psychological sequela of rape. Sequelae of traumatic brain injury include headache and dizziness, anxiety, apathy, depression, aggression, cognitive impairments, personality changes, mania, psychosis.

Some conditions may be diagnosed retrospectively from their sequelae. An example is pleurisy.

Other examples of sequelae include those following neurological injury; including aphasia, ataxia, hemi- and quadriplegia, and any number of other changes that may be caused by neurological trauma. Note that these pathologies can be related to both physical and chemical traumas, as both can cause lingering neuron damage.

The phrase "status post", abbreviated in writing as "s/p", is used to discuss sequelae with reference to their cause. Clinicians typically use the phrase to refer to acute traumatic conditions. For example: "the patient had neck pain "status post" a motor vehicle accident".

Rheumatic fever is a nonsuppurative sequela of a primary infection of group A "Streptococcus" bacteria. Glomerulonephritis can also be a sequela of "Streptococcus pyogenes".

A sequela (, ; usually used in the plural, sequelae) is a pathological condition resulting from a disease, injury, therapy, or other trauma. Typically, a sequela is a chronic condition that is a complication which follows a more acute condition. It is different from, but is a consequence of, the first condition. Timewise, a sequela contrasts with a late effect, where there is a period, sometimes as long as several decades, between the resolution of the initial condition and the appearance of the late effect.

In general, non-medical usage, the terms "sequela" and "sequelae" mean consequence and consequences.

Estimates of the rate of HCV vertical transmission range from 2–8%; a 2014 systematic review and meta-analysis found the risk to be 5.8% in HCV-positive, HIV-negative women. The same study found the risk of vertical transmission to be 10.8% in HCV-positive, HIV-positive women. Other studies have found the risk of vertical transmission to be as high as 44% among HIV-positive women. The risk of vertical transmission is higher when the virus is detectable in the mother's blood.

Evidence does not indicate that mode of delivery (i.e. vaginal vs. cesarean) has an effect on vertical transmission.

For women who are HCV-positive and HIV-negative, breastfeeding is safe; however, CDC guidelines suggest avoiding breastfeeding if a woman's nipples are "cracked or bleeding" to reduce the risk of transmission.

Pregnant women who contract HEV are at significant risk of developing fulminant hepatitis with maternal mortality rates as high as 20–30%, most commonly in the third trimester . A 2016 systematic review and meta-analysis of 47 studies that included 3968 people found maternal case-fatality rates (CFR) of 20.8% and fetal CFR of 34.2%; among women who developed fulminant hepatic failure, CFR was 61.2%.

As in the majority of paracoccidioidomycosis cases, pulmonary involvement results in shortness of breath, a productive cough and hemoptysis, as well as general symptoms of weight loss, fever and fatigue. Visually, lesions (as pictured) are often present, most commonly on the face.

"P. brasiliensis" is a thermally dimorphic fungus distributed in Brazil and South America. The habitat of the infectious agent is not known, but appears to be aquatic. In biopsies, the fungus appears as a polygemulating yeast with a pilot's wheel-like appearance.

Wissler's syndrome (or Wissler's disease or Wissler-Fanconi syndrome) is a rheumatic disease that has a similar presentation to sepsis. It is sometimes considered closely related to Still's disease. It is named for Guido Fanconi and Hans Wissler It was first described by Wissler in 1944 and Fanconi in 1946. Single observations by E. Uhse in 1943 («Febris maculosa intermittens»), Fykow in 1929 and Nowak in 1942.

Etiology uncertain. Wissler suggested an allergic reaction to bacteraemia as the pathogenic factor.

There are assertions that race affects the rate of SLE. However, a 2010 review of studies which correlate race and SLE identified several sources of systematic and methodological error, indicating that the connection between race and SLE may be spurious. For example, studies show that social support is a modulating factor which buffers against SLE-related damage and maintains physiological functionality. Studies have not been conducted to determine whether people of different racial backgrounds receive differing levels of social support. If there is a difference, this could act as a confounding variable in studies correlating race and SLE. Another caveat to note when examining studies about SLE is that symptoms are often self-reported. This process introduces additional sources of methodological error. Studies have shown that self-reported data is affected by more than just the patients experience with the disease- social support, the level of helplessness, and abnormal illness-related behaviors also factor into a self-assessment. Additionally, other factors like the degree of social support that a person receives, socioeconomic status, health insurance, and access to care can contribute to an individual’s disease progression. Racial differences in lupus progression have not been found in studies that control for the socioeconomic status [SES] of participants. Studies that control for the SES of its participants have found that non-white people have more abrupt disease onset compared to white people and that their disease progresses more quickly. Non-white patients often report more hematological, serosal, neurological, and renal symptoms. However, the severity of symptoms and mortality are both similar in white and non-white patients. Studies that report different rates of disease progression in late-stage SLE are most likely reflecting differences in socioeconomic status and the corresponding access to care. The people who receive medical care have often accrued less disease-related damage and are less likely to be below the poverty line. Additional studies have found that education, marital status, occupation, and income create a social context which contributes to disease progression.

SLE, like many autoimmune diseases, affects females more frequently than males, at a rate of about 9 to 1. The X chromosome carries immunological related genes, which can mutate and contribute to the onset of SLE. The Y chromosome has no identified mutations associated with autoimmune disease.

Hormonal mechanisms could explain the increased incidence of SLE in females. The onset of SLE could be attributed to the elevated hydroxylation of estrogen and the abnormally decreased levels of androgens in females. In addition, differences in GnRH signalling have also shown to contribute to the onset of SLE. While females are more likely to relapse than males, the intensity of these relapses is the same for both sexes.

In addition to hormonal mechanisms, specific genetic influences found on the X chromosome may also contribute to the development of SLE. Studies indicate that the X chromosome can determine the levels of sex hormones. A study has shown an association between Klinefelter syndrome and SLE. XXY males with SLE have an abnormal X–Y translocation resulting in the partial triplication of the PAR1 gene region.