COVID 19 is caused by the SARS CoV 2 virus, which primarily spreads through respiratory droplets and aerosols produced when infected people breathe, talk, cough or sneeze. Transmission occurs especially in close, indoor and poorly ventilated settings. Asymptomatic and pre symptomatic individuals can also spread the virus, making control more difficult.

Once inside the body, the virus binds mainly to ACE2 receptors on cells in the respiratory tract. Infection can cause a range of outcomes from no symptoms to severe pneumonia and multi organ failure. Typical symptoms include fever, cough, fatigue, loss of taste or smell, and breathing difficulties, though new variants have changed the most common symptom patterns over time.

Disease severity is strongly influenced by age, underlying conditions such as cardiovascular disease, diabetes, obesity and chronic lung disease, and by immune status. Older adults and immunocompromised people face the highest risk of hospitalization and death.

Preventive strategies focus on vaccination, ventilation, masks in high risk situations, hand hygiene and isolation of infected individuals. Vaccines based on mRNA and viral vectors stimulate immune memory that greatly reduces the risk of severe disease and death, although protection against infection and mild illness can wane and is affected by viral variants.

The virus continues to evolve, generating variants with altered transmissibility, immune escape and clinical characteristics. This ongoing evolution requires updated vaccines, surveillance of case and variant patterns, and flexible public health responses. Long term consequences of infection, known as long COVID, remain an active area of research, with persistent symptoms reported even after mild initial illness.