COVID 19 is caused by the SARS CoV 2 virus, which primarily infects the respiratory tract but can affect many organs through inflammation and immune responses. The virus spreads mainly through respiratory droplets and aerosols produced when an infected person breathes, speaks, coughs or sneezes. Transmission risk is highest in crowded, poorly ventilated indoor spaces and during close, prolonged contact.

Infection begins when viral particles bind to ACE2 receptors on human cells, especially in the nose and lungs. After entry, the virus uses the host cell machinery to replicate, producing large numbers of new viral particles. The incubation period is typically a few days, and people can be infectious before symptoms appear. Symptoms range from none at all to severe pneumonia, respiratory failure and multi organ involvement. Age, underlying health conditions and immune status strongly influence disease severity.

Immune responses involve both innate and adaptive components. Neutralizing antibodies and T cells help clear the virus and provide protection against reinfection, although immunity can wane over time. Viral evolution has produced multiple variants with changes in transmissibility, immune escape and disease patterns. Monitoring these variants remains essential for public health planning.

Vaccines using mRNA, viral vectors and protein subunits have significantly reduced severe disease and death. They work by training the immune system to recognize the spike protein of the virus. Public health measures, including masks, ventilation, testing, isolation of cases and vaccination campaigns, have been central to controlling spread. Ongoing research continues to refine treatments, improve vaccines and better understand long term consequences such as post acute COVID conditions.