World Pneumonia Day:
Fighting Viruses, Fighting Pneumonia

1. Influenza Virus

Influenza virus is one of the common pathogens causing viral pneumonia, the incidence of which increases significantly during the influenza season. Influenza viruses are highly contagious RNA viruses belonging to the family of Orthomyxoviridae, which are categorized into three main types: type A, type B, and type C. These viruses are transmitted by airborne droplets such as coughs or sneezes. These viruses are spread through airborne droplets, such as those released when coughing or sneezing.

The basic structure of an influenza virus consists of an outer shell, a glycoprotein, and a viral genome. The outer shell consists of a lipid bilayer membrane embedded with a variety of glycoproteins that play key roles in viral infection and immune escape.

Among them, hemagglutinin (HA) is responsible for viral binding and invasion of host cells and is the main target of influenza vaccines, while neuraminidase (NA) assists in the release of the virus within the host cell and facilitates viral transmission. The viral genome consists of eight segments of RNA, each encoding different proteins, such as components of the viral polymerase complex (PB2, PB1, PA), and nucleoprotein (NP), etc. This segmented structure allows influenza viruses to quickly adapt to the host's immune response during infection.

Figure 1. Structure of the influenza virus

Currently, there are two main mechanisms of treatment against influenza virus: antiviral drugs and vaccination. Antiviral drugs such as neuraminidase inhibitors (oseltamivir) and polymerase inhibitors slow down the spread of the virus by inhibiting its replication and transcription processes. Vaccination helps the host to resist influenza virus infection by inducing an immune response against HA and NA. However, due to the mutability of influenza viruses, the effectiveness of the vaccine can be somewhat compromised, and regular updating of the vaccine strain is required to cope with changes in the prevalent strain.

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2. Coronaviruses

Coronaviruses are a class of linear single-stranded positive-stranded RNA viruses with capsid encapsidation, which are widely found in nature and are capable of causing respiratory infections of varying degrees of severity. Coronaviruses are divided into four genera: α, β, γ, and δ. They are currently found to infect only vertebrates, and are prone to cause zoonotic diseases, which are a major risk to public health safety. Among them, the β genus of coronaviruses includes Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV), Middle East Respiratory Syndrome Coronavirus (MERS-CoV), and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), all of which are capable of infecting human beings and causing severe respiratory diseases.

SARS-CoV-2 is the most recent global pandemic caused by a coronavirus, and since its first discovery in late 2019, it has rapidly spread to multiple countries or regions and territories across the globe, leading to an outbreak of novel coronavirus pneumonia (COVID-19).The rapid spread of SARS-CoV-2, its high contagiousness, and the general lack of immunity in the population to it pose a significant global public health security. In addition, new mutant strains of SARS-CoV-2 have emerged, making the situation of epidemic prevention and control more complicated.

Figure 2. Structure of the coronaviruses

The coronavirus genome encodes a spiny protein (S), an envelope protein, a membrane protein, and a nuclear protein in the following order. Of these, the spiny protein is the most important surface membrane protein of coronaviruses. The mechanism of infection of coronaviruses is mainly through the interaction of their surface S-protein with the human ACE2 receptor to infect lung epithelial cells. After viral infection, the host's immune system triggers an immune response to the viral infection, but abnormalities in the immunoregulatory mechanism can cause an uncontrolled immune response, leading to cellular damage in lung tissue and impaired lung function. Therefore, understanding the interaction between coronaviruses and the host immune system is essential for controlling and eliminating viral infections.

Currently, treatments for coronavirus are mainly focused on improving symptoms, including the use of antiviral therapeutic agents such as alpha-interferon, lopinavir/ritonavir, and raltegravir. Meanwhile, traditional Chinese medicine has shown significant antiviral effects and fewer adverse effects in the prevention and treatment of coronavirus infections. In addition, the development of inactivated vaccines is one of the important strategies to deal with the coronavirus pandemic.

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3. Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is an enveloped virus belonging to the subfamily Pneumovirus of the family Paramyxoviridae, with two subtypes, A and B. RSV viral particles can be spherical, filamentous, or irregular in shape, with diameters ranging from 100 nm to 1000 nm. The complex structure includes a single-stranded negative-stranded RNA genome and a variety of key proteins, such as fusion protein (F protein), adsorption protein (G protein), matrix protein (M protein), nucleocapsid protein (N protein), and M2-1 protein, etc. These proteins play key roles in the assembly, replication, and infection of the virus.

Figure 2. Structure of the respiratory syncytial virus

RSV is spread primarily through droplet and direct contact, and RSV infections are very common in children, especially in winter and early spring. Children infected with RSV may experience symptoms ranging from mild upper respiratory symptoms to severe lower respiratory tract infections such as bronchiolitis and pneumonia. In severe cases, RSV infection may lead to respiratory failure and other complications.

Currently, there are no specific antiviral therapeutic agents for RSV, and treatment is primarily symptomatic and supportive. However, in recent years, hope has been raised for the prevention of RSV as several RSV vaccines and monoclonal antibodies have entered phase III clinical trials.