Influenza virus infection, one of the most common infectious diseases, is a highly contagious airborne disease that causes an acute febrile illness and results in variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. These symptoms contribute to significant loss of workdays, human suffering, mortality, and significant morbidity. Influenza results from infection with 1 of 3 basic types of influenza virus: A, B, or C which are classified within the family Orthomyxoviridae. These single-stranded RNA viruses are structurally and biologically similar but vary antigenically. The most common prevailing influenza A subtypes that infect humans are H1N1, H5N1 and H3N2.

Chlamydophila (formerly Chlamydia) pneumoniae causes mild pneumonia or bronchitis in adolescents and young adults. Older adults may experience more severe disease and repeated infections. Approximately 50% of young adults and 75% of elderly persons have serological evidence of previous infection. The pathogen is estimated to cause 10-20% of community-acquired pneumonia cases among adults. The estimated number of cases of C pneumoniae pneumonia is 300,000 cases per year.

Pseudomonas is a gram-negative rod that belongs to the family Pseudomonadaceae. Pseudomonas aeruginosa has become an important cause of infection, especially in patients with compromised host defense mechanisms. Pseudomonal bacteremia occurs in association with malignancy, chemotherapy, AIDS, burn wound sepsis, and diabetes. Predisposing conditions include placement of intravenous lines, severe burns, urinary tract catheterization, surgery, trauma, and premature birth. It is the most common pathogen isolated from patients who have been hospitalized longer than 1 week. It is a frequent cause of nosocomial infections such as pneumonia, endocarditis, meningitis, urinary tract infections (UTIs) and bacteremia.

Legionella pneumophila is a thin, pleomorphic, flagellated Gram-negative bacterium of the genus Legionella. Legionella pneumophila is ubiquitous to acquatic environments worldwide and resided as an intracellular parasite of amoeba and protozoa provided a link between natural environment and human disease. Thus, environmental monitoring, especially of potable water, cooling towers, and related sources, is a major focus in efforts to control the spread of this disease. Legionnaires disease is the most common cause of atypical pneumonia in hospitalized patients. It is the second most common cause of community-acquired bacterial pneumonia with 25% mortality rate.

Human respiratory syncytial virus (hRSV) is a negative-sense, single-stranded RNA virus of the family Paramyxoviridae, the most common cause of severe lower respiratory tract disease among infants and young children, typically infects persons by age 2 years and can cause subsequent infections throughout life. RSV infection primarily manifests as bronchiolitis or pneumonia and results in approximately 75,000 to 125,000 hospitalizations in the United States each year. Persons at increased risk for severe disease or death include premature infants, older adults, and persons of any age with compromised respiratory, cardiac, or immune systems.

Acute respiratory viral infections (ARVI) are the most frequent illnesses experienced by most people globally. Young children have between five and seven of these illnesses per year, with a general decrease in frequency with increasing age. Adults will have about two such illnesses per year.
A wide variety of viruses, containing both RNA and DNA, cause acute respiratory infections. The most important viruses are rhinoviruses, coronaviruses, parainfluenza viruses, respiratory syncytial virus, adenoviruses, metapneumoviruses.

Bordetella pertussis is an obligate human pathogen and is the causative agent of whooping cough (pertussis). Bordetella parapertussis causes a milder form of disease in humans and also causes respiratory infections in sheep. Bordetella bronchiseptica causes disease in many mammalian species, but humans, however, are rarely infected with this organism and typically involve immunocompromised patients.

The novel coronavirus (SARS-CoV-2) is a new strain which has not previously been identified in humans. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), known also as 2019 novel coronavirus (2019-nCoV), is a positive-sense single-stranded RNA virus. It is contagious in humans and is the cause of the ongoing pandemic of coronavirus disease 2019 (COVID-19) that has been designated a Public Health Emergency of International Concern by the World Health Organization (WHO).
The earliest case of infection currently known is thought to have been found on November 2019. The virus subsequently spread to all provinces of China and to more than one hundred other countries in Asia, Europe, North America, South America, Africa, and Oceania. On 11 March the WHO has officially declared it a pandemic.