Virus, particularly respiratory viruses, can inhibit your immune system to evade detection. This along with low viral titers contributes to the incubation phase of a viral infection (pre-symptomatic). The viruses continues to replicate unhindered until it can no longer suppress your immune system usually due to accumulation of viral particles, host cell death, or deleterious mutations in the viral genetic material during rapid replication. The release of inhibition is when you begin to feel sick as your body quickly ramps up your immune response. You can think of this like someone holding a door shut while you try to open it and they suddenly walk away.

Another factor is that your immune system can increase activation exponentially at first. Every tissue in your body has resident immune cells that can first sound the alarm. This will trigger a flood of new immune cells which then see an active infection and also signal for more help (via cytokines). Sometimes this response can be so intense that it can be life threatening which is in part why some very severe COVID cases are treated with steroids which actually inhibit the immune system.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3185581