Table 1.

Viruses commonly implicated in the exacerbation of asthma

Name	Classification	Virus structure	Genetic material	Clinical disease	Viral proteins that directly modulate host immune response
RV	Family: Picornaviridae	Protein capsid with ‘canyon’ binding site. The canyon regions contain the binding site of the cellular receptor, ICAM-1	Non-segmented single molecule of linear positive sense RNA	Most frequent cause of common cold (responsible for 30–50% of cases). Second most common virus to cause bronchiolitis. Incidence highest in children <5 years	Not known
	Genus: Rhinovirus		Genome (7.2 kb)
	Species: Human rhinovirus
RSV	Family: Paramyxoviridae	Enveloped with F and G surface proteins. F protein: virus penetration and syncytium formation. G protein: virus attachment	Non-segmented single molecule of linear negative sense RNA	Major cause of lower respiratory tract infection in young children <5 years. Major cause of pneumonia and bronchiolitis in infants under 1 year of age	NS1 and NS2 proteins—interference of interferon system. G protein: Chemokine mimicry—inhibits the recruitment of cytotoxic cells. Drives Th2 immune response
	Genus: Pneumovirus		Genome (15 kb)
	Species: Human respiratory syncytial virus
hMPV	Family: Paramyxoviridae	Pleomorphic, enveloped virus with helical nucleocapsid and F and G surface proteins. Similar to RSV, hMPV F and G proteins may be required for infection	Non-segmented single molecule of linear negative sense RNA	Significant cause of lower respiratory tract infection in young children <5 years. Clinical disease similar to RSV	Not known
	Genus: Metapneumovirus		Genome (13 kb) smaller than other pneumoviruses such as RSV
	Species: Human metapneumovirus
Influenza virus	Family: Orthomyxoviridae	Enveloped with haemagglutinin (H) and Neuraminidase (N) proteins. Influenza A viruses are divided into subtypes based on these two surface proteins	8 segments of negative strand RNA	Major cause of respiratory infection. In serious cases, influenza causes pneumonia	NS1—interference of interferon system
	Genus: Influenza virus		Genome (12–14 kb)
	Species: Influenza virus A
	Influenza virus B
	Influenza virus C

F, fusion glycoprotein; G, G attachment glycoprotein; H, haemagglutinin; hMPV, human metapneumovirus; N, neuraminidase; ICAM-1, intracellular cell adhesion molecule 1; NS, non-structural; RV, rhinovirus; RSV, respiratory syncytial virus.