Here is a VERY good COVID-19 analysis by Ivor Cummins. All science and data, and supports the idea that the COVID problem is WAY overblown. Lockdowns don't do much at all. Well worth the 24 minutes it takes to watch. Be informed. (European data but some US data is included)

https://www.youtube.com/watch?v=cSKjcltDkng

>>9864158

Assumptions are common in physics problems, to make the math less complex. They are usually stated, as in this case, you make you aware that certain reasonable assumptions have been made. It's a tragedy what's happened to science education.