It has been hypothesized that biofilm bacterial infections may account for many cases of antibiotic-refractory chronic sinusitis. Biofilms are complex aggregates of extracellular matrix and inter-dependent microorganisms from multiple species, many of which may be difficult or impossible to isolate using standard clinical laboratory techniques. Bacteria found in biofilms have their antibiotic resistance increased up to 1000 times when compared to free-living bacteria of the same species. A recent study found that biofilms were present on the mucosa of 75% of people undergoing surgery for chronic sinusitis.

By definition chronic sinusitis lasts longer than 12 weeks and can be caused by many different diseases that share chronic inflammation of the sinuses as a common symptom. Symptoms of chronic sinusitis may include any combination of the following: nasal congestion, facial pain, headache, night-time coughing, an increase in previously minor or controlled asthma symptoms, general malaise, thick green or yellow discharge, feeling of facial 'fullness' or 'tightness' that may worsen when bending over, dizziness, aching teeth, and/or bad breath. Each of these symptoms has multiple other possible causes, which should be considered and investigated as well. Often chronic sinusitis can lead to anosmia, the inability to smell objects. In a small number of cases, acute or chronic maxillary sinusitis is associated with a dental infection. Vertigo, lightheadedness, and blurred vision are not typical in chronic sinusitis and other causes should be investigated.

Chronic sinusitis cases are subdivided into cases with polyps and cases without polyps. When polyps are present, the condition is called chronic hyperplastic sinusitis; however, the causes are poorly understood and may include allergy, environmental factors such as dust or pollution, bacterial infection, or fungus (either allergic, infective, or reactive).

Chronic rhinosinusitis represents a multifactorial inflammatory disorder, rather than simply a persistent bacterial infection. The medical management of chronic rhinosinusitis is now focused upon controlling the inflammation that predisposes patients to obstruction, reducing the incidence of infections. However, all forms of chronic rhinosinusitis are associated with impaired sinus drainage and secondary bacterial infections. Most individuals require initial antibiotics to clear any infection and intermittently afterwards to treat acute exacerbations of chronic rhinosinusitis.

A combination of anaerobic and aerobic bacteria are detected in conjunction with chronic sinusitis. Also isolated are "Staphylococcus aureus" (including methicilin resistant "S.aureus" ) and coagulase-negative "Staphylococci" and Gram negative enteric organisms can be isolated.

Attempts have been made to provide a more consistent nomenclature for subtypes of chronic sinusitis. The presence of eosinophils in the mucous lining of the nose and paranasal sinuses has been demonstrated for many patients, and this has been termed eosinophilic mucin rhinosinusitis (EMRS). Cases of EMRS may be related to an allergic response, but allergy is not often documented, resulting in further subcategorization into allergic and non-allergic EMRS.

A more recent, and still debated, development in chronic sinusitis is the role that fungi play in this disease. It remains unclear if fungi are a definite factor in the development of chronic sinusitis and if they are, what the difference may be between those who develop the disease and those who remain free of symptoms. Trials of antifungal treatments have had mixed results.

Recent theories of sinusitis indicate that it often occurs as part of a spectrum of diseases that affect the respiratory tract ("i.e.", the "one airway" theory) and is often linked to asthma. All forms of sinusitis may either result in, or be a part of, a generalized inflammation of the airway, so other airway symptoms, such as cough, may be associated with it.

Both smoking and secondhand smoke are associated with chronic rhinosinusitis.

Adenoiditis occurs mainly in childhood, often associated with acute tonsillitis. Incidence decreases with age, with adenoiditis being rare in children over 15 years due to physiological atrophy of the adenoid tissue.

Viruses that may cause adenoiditis include adenovirus, rhinovirus and paramyxovirus. Bacterial causes include Streptococcus pyogenes, Streptococcus pneumoniae, Moraxella catarrhalis and various species of Staphylococcus including Staphylococcus aureus.

Pharyngitis may also be caused by mechanical, chemical or thermal irritation, for example cold air or acid reflux. Some medications may produce pharyngitis such as pramipexole and antipsychotics.

Some cases of pharyngitis are caused by fungal infection such as Candida albicans causing oral thrush.

There is low or very-low quality evidence that probiotics may be better than placebo in preventing acute URTIs. Vaccination against influenza viruses, adenoviruses, measles, rubella, "Streptococcus pneumoniae", "Haemophilus influenzae", diphtheria, "Bacillus anthracis", and "Bordetella pertussis" may prevent them from infecting the URT or reduce the severity of the infection.

Children have 2-9 viral respiratory illnesses per year. In 2013 18.8 billion cases of upper respiratory infections were reported. As of 2014, upper respiratory infections caused about 3,000 deaths down from 4,000 in 1990. In the United States, URIs are the most common infectious illness in the general population. URIs are the leading reasons for people missing work and school.

Rhinorrhea can also occur when individuals with allergies to certain substances, such as pollen, dust, latex, soy, shellfish, or animal dander, are exposed to these allergens. In people with sensitized immune systems, the inhalation of one of these substances triggers the production of the antibody immunoglobulin E (IgE), which binds to mast cells and basophils. IgE bound to mast cells are stimulated by pollen and dust, causing the release of inflammatory mediators such as histamine. In turn, this causes, among other things, inflammation and swelling of the tissue of the nasal cavities as well as increased mucus production. Particulate matter in polluted air and chemicals such as chlorine and detergents, which can normally be tolerated, can make the condition considerably worse.

Rhinorrhea can be a symptom of other diseases, such as the common cold or influenza. During these infections, the nasal mucous membranes produce excess mucus, filling the nasal cavities. This is to prevent infection from spreading to the lungs and respiratory tract, where it could cause far worse damage. It has also been suggested that rhinorrhea is a result of viral evolution, and may be a response that is not useful to the host, but which has evolved by the virus to maximise its own infectivity. Rhinorrhea caused by these infections usually occur on circadian rhythms. Over the course of a viral infection, sinusitis (the inflammation of the nasal tissue) may occur, causing the mucous membranes to release more mucus. Acute sinusitis consists of the nasal passages swelling during a viral infection. Chronic sinusitis occurs when one or more nasal polyps appear. This can be caused by a deviated septum as well as a viral infection.

Though it is widely held that fungal infections of the nose and paranasal sinuses are not common, most agree that their frequency has been increasing over past decades.

Treatment for fungal sinusitis can include surgical debridement; helps by slowing progression of disease thus allowing time for recovery additionally we see the options below:

- In the case of invasive fungal sinusitis, echinocandins, voriconazole, and amphoterecin (via IV) may be used

- For allergic fungal sinusitis, systemic corticosteroids like prednisolone, methylprednisolone are added for their anti-inflammatory effect, bronchodilators and expectorants help to clear secretions in the sinuses.

Bacterial infections of the orbit have long been associated with a risk of catastrophic local

sequelae and intracranial spread.

The natural course of the disease, as documented by Gamble (1933), in the pre-antibiotic era,

resulted in death in 17% of patients and permanent blindness in 20%.

Due to the human ear's function of regulating the pressure within the head region, catarrh blockage may cause discomfort during changes in atmospheric pressure.

First-generation antihistamine has been suggested as first-line therapy to treat post-nasal drip.

PND is suggested to be a cause of extra-oral halitosis, especially when a sinus infection is also present. Acid reflux or heartburn is believed to aggravate and in some cases cause post-nasal drip. Post-nasal drip can be a cause of laryngeal inflammation and hyperresponsiveness, leading to symptoms of vocal cord dysfunction (VCD).

Catarrh , or catarrhal inflammation, is inflammation of the mucous membranes in one of the airways or cavities of the body, usually with reference to the throat and paranasal sinuses. It can result in a thick exudate of mucus and white blood cells caused by the swelling of the mucous membranes in the head in response to an infection. It is a symptom usually associated with the common cold, pharyngitis, and chesty coughs, but it can also be found in patients with adenoiditis, otitis media, sinusitis or tonsillitis. The phlegm produced by catarrh may either discharge or cause a blockage that may become chronic.

The word "catarrh" was widely used in medicine since before the era of medical science, which explains why it has various senses and in older texts may be synonymous with, or vaguely indistinguishable from, common cold, nasopharyngitis, pharyngitis, rhinitis, or sinusitis. The word is no longer as widely used in American medical practice, mostly because more precise words are available for any particular pathosis. Indeed, to the extent that it is still used, it is no longer viewed nosologically as a disease entity but instead as a symptom, a sign, or a syndrome of both. The term "catarrh" is found in medical sources from the United Kingdom. The word has also been common in the folk medicine of Appalachia, where medicinal plants have been used to treat the inflammation and drainage associated with the condition.

Complications include hearing loss, blood infection, meningitis, cavernous sinus thrombosis, and optic nerve damage (which could lead to blindness).

Normal surgical masks and N95 masks appear equivalent with respect to preventing respiratory infections.

Most household disinfectants will inactivate FHV-1. The virus can survive up to 18 hours in a damp environment, but less in a dry environment and only shortly as an aerosol.

A 2014 systematic review of clinical trials does not support using routine rapid viral testing to decrease antibiotic use for children in emergency departments. It is unclear if rapid viral testing in the emergency department for children with acute febrile respiratory infections reduces the rates of antibiotic use, blood testing, or urine testing. The relative risk reduction of chest x-ray utilization in children screened with rapid viral testing is 77% compared with controls. In 2013 researchers developed a breath tester that can promptly diagnose lung infections.

Sinusitis is inflammation of the paranasal air sinuses. Infections associated with teeth may be responsible for approximately 20% of cases of maxillary sinusitis. The cause of this situation is usually a periapical or periodontal infection of a maxillary posterior tooth, where the inflammatory exudate has eroded through the bone superiorly to drain into the maxillary sinus. Once an odontogenic infection involves the maxillary sinus, it is possible that it may then spread to the orbit or to the ethmoid sinus.

Antibiotics are aimed at gram positive bacteria. Medical attention should be sought if symptoms persist beyond 2–3 days.

There is a vaccine for FHV-1 available (ATCvet code: , plus various combination vaccines), but although it limits or weakens the severity of the disease and may reduce viral shedding, it does not prevent infection with FVR. Studies have shown a duration of immunity of this vaccine to be at least three years. The use of serology to demonstrate circulating antibodies to FHV-1 has been shown to have a positive predictive value for indicating protection from this disease.

"Staphylococcus aureus", "Streptococcus pneumoniae", other streptococci, and anaerobes are the most common causes, depending on the origin of the infection.

The advent of the "Haemophilus influenzae" vaccine has dramatically decreased the incidence.