**5.5. Herpes simplex virus**

Herpes simplex virus (HSV) is primarily implicated in severely immunocompromised patients primarily, e.g., solid organ transplant recipients, patients who are undergoing chemotherapy or are neutropenic, or those who have congenital immunodeficiency. Herpes simplex virus is spread by viral shedding from asymptomatic excreters or from active lesions. It is a rare cause of lower respiratory tract infections. HSV pneumonia can occur as a secondary infection to upper airway infection or following viremia due to genital or oral lesions [44]. Herpes simplex virus can also cause pneumonia in compromised hosts, with a mortality rate of 80%.

Herpes simplex virus type 1 pneumonia is relatively uncommon that generally affects patients who are immunocompromised. It often occurs as a polymicrobial infection and is frequently associated with coexisting bacterial pneumonia. Pneumonia is usually characterized by a proteinaceous exudate and alveolar necrosis. There is a variable polymorphonuclear inflammatory response [45].

the respiratory route and illness begins with fever, cough, coryza. Complications of measles affect most organ systems, with pneumonia accounting for most measles-associated morbidity

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Pulmonary disease from measles virus infection can occur as a primary measles virus pneumonia with secondary bacterial pneumonia or as an atypical measles virus pneumonia. Measles virus can cause pneumonia in 3–4% of infected patients mostly with secondary bacterial infection such as Haemophilus influenzae and Neisseria meningitides. The prevalence of measles virus pneumonia is higher in immunocompromised patients and pregnant women. Measles virus pneumonia without a secondary bacterial infection appears with diffuse alveolar damage and epithelial hyperplasia. Epithelial hyperplasia is seen in bronchioles and peribronchial alveoli as well as in the tracheobronchial epithelium with cystic dilatation of mucous glands. Histologically, measles virus pneumonia displays multinucleated giant cells containing up to

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.

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

Respiratory syncytial virus (RSV) consists of only one serotype and is in the Paramyxoviridae family. Structurally, it consists of 10 viral polypeptides, 4 of which are associated with virus envelope, and 2 of these (F and G) are important for infectivity and pathogenicity. RSV is highly contagious, spreading via droplet and fomite exposure. RSV is the most frequent cause of lower respiratory tract infections among infants and children and the second most common viral cause of pneumonia in adults [54]. The majority of children are infected by the age of 5 years in settings such as daycare centers but the resulting immunity is incomplete. Reinfection when it occurs in older children and young adults is mild. But, with advancing age there is a greater likelihood severe disease and pneumonia. Diagnosed adult RSV hospitalizations have increased significantly in the United States. Respiratory syncytial virus hospitalizations appear to be greater in severity than influenza hospitalizations, especially

50 nuclei within the bronchiolar and tracheobronchial epithelium [51].

Transmission is through direct contact or large-droplet spread.

pneumonia there was a mortality rate of 37% [53].

immunocompromised and in older adults [55].

**5.10. Respiratory syncytial virus**

and mortality.

**5.9. Parainfluenza virus**

#### **5.6. Human metapneumovirus**

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 exposure.

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