COVID-19 symptoms vary from person to person, and so does the time it takes for the symptoms to appear. In general, it may take between two to 14 days after exposure to COVID-19 for symptoms to appear. According to the WHO, on average it may take 5-6 days for symptoms to appear.

Symptom onset also depends on the variant to which one is exposed.The incubation period for the Delta variant was shorter than the previous variants, and for Omicron it appears to be about three days, on average.

Some people have mild to moderate symptoms including fever, cough, tiredness, loss of taste or smell, sore throat, aches, and pains, while others may have more serious symptoms. Others may be asymptomatic, and infected people without symptoms (pre-symptomatic or asymptomatic) can still infect others and are most infectious during the two days before symptoms appear. It is recommended that public health guidance be followed and one get tested if they have been exposed to anyone who has been confirmed to be infected with Sars-CoV-2.

The period between exposure to a germ and when the symptoms show up is called the incubation period. It varies for different conditions and may vary from person to person. One is still infectious during the incubation period, that is, they can easily infect others before they start showing symptoms.

Scientists are trying to understand the phases of COVID-19 infection from various theoretical and clinical perspectives. There is no definitive or globally accepted classification of the stages of COVID-19 infection.

Several studies have classified the infection into three stages. 1) The pre-symptomatic or asymptomatic phase (early infection) 2) The propagation of the virus to the upper airways and respiratory tracts that lead to mild, moderate, or severe symptoms 3) The multi-system impairment with hyper inflammation that may injure multiple organs.

In other words, these can be categorized as the viral infection stage, vascular or pulmonary inflammation stage, and fibrosis stage. These stages might occur consecutively or overlap one another. 

One study classifies the stages into pre-exposure, incubation, detectable viral replication, followed by five more phases that end with a tail phase. Another study on non-severe COVID-19 patients proposes four immunological phases: prodromal phase (in the first week), the apparent manifestation phase (in the second week), the remission phase (in the third week), and the convalescent phase (after 3 weeks).