**5.9. Parainfluenza virus**

associated with coexisting bacterial pneumonia. Pneumonia is usually characterized by a proteinaceous exudate and alveolar necrosis. There is a variable polymorphonuclear inflammatory

Human metapneumovirus (hMPV) is in the Paramyxoviridae family and was initially described in the Netherlands in 2001 [46]. It is a pleomorphic-shaped virus with protein projections from the surface. hMPV is a ubiquitous organism and almost all children have been exposed to it by age 5 years, Morbidity in lower respiratory tract infections in children and infants was reported to bronchiolitis (59%), croup (18%), asthma (14%), and pneumonia (8%) [47]. Reinfection continues to occur throughout life. The virus is spread via droplet and fomite

The severity of infection increases with older age and with complications such as immunosuppressive conditions or cardiopulmonary disease. Adult hospitalizations with hMPV infection are associated with chronic obstructive pulmonary disease (COPD) exacerbations, pneumonia and bronchitis. Severe pneumonitis requiring intensive care is required in immu-

The influenza viruses are RNA viruses in the family Orthomyxoviridae, they are enveloped, single-stranded, and are the most common viral cause of pneumonia. Influenza has three serotypes A, B, and C. Influenza type A can infect livestock allowing a reservoir for infection and opportunity for epidemics in humans. For this reason it is usually the most virulent pathogen. The structure of influenza virus includes two envelope glycoproteins, known as hemagglutinin (H) and neuraminidase (N).The hemagglutinin enables infectivity of the virus by attaching to cellular sialic acid residues, whereas the N protein allows spread of the virus to other cells through cleavage of the new virus. Severe pneumonia complications can occur in high-risk individuals [48]. Two influenza types have emerged of particular importance: H5N1 avian influenza strain and the novel H1N1 swine influenza strain. In the influenza A (H1N1) pandemic of 2009–2010, the World Health Organization estimated approximately 16,000 deaths. The majority of these deaths corresponded to patients with underlying risk factors contributing to worse outcomes Influenza type B causes illness seems to occur more in closed populations, e.g., boarding schools. Influenza type C is less common and occurs as

Measles is a respiratory tract virus that causes a febrile illness with rash in children and a mild pneumonia in healthy adults. It is a single-stranded RNA virus in the Paramyxoviridae family and the genus *Morbillivirus*. It comprises a nucleocapsid surrounded by an envelope. Measles is a highly contagious disease that results from infection with measles virus and is still responsible for more than 100,000 deaths every year [50]. Measles virus is transmitted by

response [45].

198 Contemporary Topics of Pneumonia

exposure.

**5.7. Influenza virus**

sporadic cases [49].

**5.8. Measles virus**

**5.6. Human metapneumovirus**

nocompromised hosts (e.g., hematologic malignancies).

Parainfluenza virus (PIV) consists of nucleocapsids, which propagate in the cytoplasm of infected cells, with hemagglutinin present in the virion envelope. PIV is a common virus infection of childhood. PIV is second in importance to only RSV in causing children pneumonia and bronchiolitis in infants younger than 6 months and lower respiratory tract disease. Transmission is through direct contact or large-droplet spread.

Although there are four subtypes of PIV, PIV types 1 and 2 tend to peak during the fall season where as type 3 is endemic year-round. Recurrent upper or lower respiratory tract infections occur throughout life because Immunity is short term. The infections vary from a self-limiting illness to life-threatening pneumonia especially in immunocompromised hosts leading to lung injury and respiratory failure [52]. In one study, hematopoietic stem cell transplant (HSCT) patients with PIV progressed to develop pneumonia. Of 44% of these patients with pneumonia there was a mortality rate of 37% [53].
