October through May is deemed “cold and flu season.” The common cold and flu are both respiratory illnesses but they are caused by different viruses. Rhinovirus (RV) is the most common virus involved in the cold while flu viruses mainly include influenza A and B. Although, these two types of illnesses have similar symptoms such as sneezing, runny nose, nasal congestion, and sore throat, the flu is worse than the common cold, and symptoms are more intense. Colds are usually milder than flu. Symptoms of the flu typically come on very suddenly and include fever, chills, cough, sore throat, fatigue, headache, muscle, and body aches.
RV infections are the primary cause of the common cold. RVs are non-enveloped with a positive-sense single-stranded RNA ((+)ssRNA) genome that is protected by an icosahedral protein capsid. The common cold is a viral infection of the upper respiratory tract involving the nose and can include the sinuses, ears, bronchial tubes, and eustachian tubes. The virus enters into the nasal mucosa where it binds to the respective receptor at the ciliated cell surface. Once the virus enters the cell and begins the replication process, the immune system induces a cytokine expression. A runny nose and nasal obstruction are the most prominent symptoms that are associated with a neutrophilic inflammatory response due to increased vascular permeability and stimulation of mucus hypersecretion.1
Studies demonstrate that rhinovirus (RV) by itself does not cause damage to the airway epithelial barrier like other viruses do e.g. influenza virus. 1 However, RV infection does disrupt epithelial barrier function as well as the tight junction which in turn facilitates the translocation of pathogens and their soluble products.
Lower respiratory symptoms associated with RV infection are most prominent in patients who have underlying asthma or other chronic lung diseases. RV is one of the most common triggers for acute exacerbations in those with asthma and chronic lung conditions. These symptoms include cough, shortness of breath, chest tightness, and wheezing.
Influenza, commonly known as "the flu", is caused by an influenza virus. Influenza viruses are negative sense single-stranded RNA ((-) ssRNA) viruses. Influenza viruses cause acute respiratory inflammation and symptoms such as cough, headache, high fever, body aches, and fatigue. There are four types of influenza viruses: A, B, C and D. Influenza A viruses (IAV), together with influenza B viruses, cause respiratory illness in humans. Some people, such as older people, young children, and people with certain health conditions, are at high risk of serious flu complications such as pneumonia. Serious outcomes of flu infection can result in hospitalization or death.
IAV and B virus cause seasonal epidemics of disease, known as the flu season, almost every winter in the United States. IAV are the only influenza viruses known to cause global flu pandemics. IAV are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). While there are potentially 198 different IAV subtype combinations, only 131 subtypes have been detected in nature. Current subtypes of IAV that routinely circulate in people include A(H1N1) and A(H3N2). The most outstanding characteristic of the influenza viruses is their rapid evolution which leads to its great variability. This is the case especially with IAV and therefore it is the most feared type of influenza virus.
Following the infection, the host immune system aims to defend against and clear the viral infection. The IAV first induces the innate immune system to recruit various phagocytic cells, groups of cytokines, and interferons (IFNs) to the site of infection.3 The adaptive immunity is mediated by B cells and T cells mainly the CD4+ and CD8+ T cells capturing and neutralizing the pathogen. The humoral immune response functions through hemagglutinin-specific circulating antibodies to neutralize IAV. In addition, antibodies can bind to the surface of infected cells and induce antibody-dependent cell-mediated cytotoxicity or complement activation.3
Complications or ultimately death arising from these infections are often associated with hyper-induction of pro-inflammatory cytokine production, which is also known as ‘cytokine storm'. Cytokine storm is associated with uncontrolled pro-inflammatory responses which are complicated by several types of cytokines and chemokines that have various activities. In addition to their direct effects, their cross-regulation causes cytokine network to form which determines the outcome of viral infections.4 Antibody induced cytotoxicity/complement activation can lead to the airway epithelial barrier destruction. Autopsy studies revealed that severe infection with influenza damages the airway and alveolar epithelium.4
Bronchitis and Pneumonia
The same viruses that cause colds and the flu often cause acute bronchitis and pneumonia. Bronchitis is the inflammation of the bronchial tubes while pneumonia causes inflammation of the alveoli.
In bronchitis, when the viral infection spreads form the upper respiratory tract to the lower respiratory tract, it triggers the inflammation of the bronchial walls which leads to mucosal thickening, epithelium-cell desquamation, and denudation of the basement membranes.2
Much like bronchitis, people with pneumonia will experience a cough which brings up mucus, as well as a shortness of breath. Pneumonia may similarly be accompanied by a fever – although the fever may be high, unlike bronchitis. Pneumonia affects how air is distributed to blood cells. When cells do not get enough oxygen, they cannot function properly. As a result, the infection may spread and become deadly. In an individual who suffers from a chronic lung condition, pneumonia can be very dangerous as it exacerbates the condition greatly.