Chickenpox
Primary varicella-zoster infection spreads by viraemia to the skin in successive waves, so lesions at different stages coexist at any one time, and the virus's retreat into dorsal root ganglia rather than true clearance is what allows it to return later as shingles.
First principles
The rash appears in crops because the virus reaches the skin by successive waves of viraemia
Varicella-zoster virus is inhaled or transferred by contact, replicates in the respiratory mucosa and regional lymph nodes, and then seeds the skin via a primary and then secondary viraemia. Because the virus reaches the skin in successive waves over several days rather than all at once, new lesions keep appearing while earlier ones progress through their own lifecycle: this is why the hallmark of chickenpox is lesions at multiple different stages (macules, papules, vesicles, pustules, crusts) present simultaneously, rather than a rash that is uniform in appearance.
Educational content pending clinical review. Not medical advice.