**5.7. Influenza virus**

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 sporadic cases [49].

#### **5.8. Measles virus**

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 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 and mortality.

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