Alright, here is the SEIR model - and look, something new pops up! The yellow curve rises before the red wave peaks. That is people who already caught the bug but are not contagious yet. That is the incubation period. Then the second part feels familiar. Green drops as fewer can catch it, and blue climbs with all the recoveries. So it is like the SIR model you have seen before, just with this extra stage in between.
