COVID-19 develops when a person becomes infected with SARS-CoV-2, a novel coronavirus believed to have originated in animals before spreading to humans, likely at a seafood and live animal market in Wuhan, China in December 2019.
Once the virus enters the body, the following occurs:
- Entry: SARS-CoV-2 typically enters the body through the respiratory tract (nose, mouth, or eyes) after exposure to respiratory droplets or aerosols containing the virus. These droplets are produced when an infected person coughs, sneezes, talks, or sings.
- Infection of cells: The virus uses its spike protein to bind to ACE2 receptors on the surface of human cells, primarily in the respiratory system (lungs, nose, and throat). ACE2 receptors are also found in other organs, explaining the diverse symptoms.
- Replication: Once inside the cell, the virus hijacks the cell's machinery to replicate itself, producing numerous copies of the virus.
- Spread: The newly produced virus particles are released from the infected cell, spreading to infect more cells within the respiratory system and potentially other parts of the body.
- Immune Response: The body's immune system recognizes the virus as a foreign invader and mounts a defense. This includes:
- Innate Immune Response: This is the body's first line of defense and includes inflammation, fever, and the activation of immune cells like macrophages and natural killer cells.
- Adaptive Immune Response: This involves the production of antibodies that specifically target the virus and the activation of T cells that kill infected cells.
- Disease Progression: The severity of COVID-19 depends on several factors, including the amount of virus a person is exposed to, the individual's immune response, and underlying health conditions. The disease can range from asymptomatic infection to mild respiratory illness to severe pneumonia, acute respiratory distress syndrome (ARDS), organ failure, and death.
- Transmission: An infected person can transmit the virus to others even before they develop symptoms (presymptomatic transmission) or if they never develop symptoms (asymptomatic transmission). The virus is spread through respiratory droplets and aerosols, which can be inhaled by others or land on surfaces.
The development of COVID-19 is a complex process involving viral entry, replication, spread, and the body's immune response. The interaction of these factors determines the severity and outcome of the infection.
