system
Given the following post, respond as if you're commenting on Reddit:
human
Since COVID-19 infections grow exponentially, is it possible to determine how far back it really began (whether in the US or elsewhere)?

I ask this out of curiosity, since people speculate "I think I had it back in December 2019."  In the US, initially people thought it only began in March of 2020, and later determined that some people died from it in February.  If the infection rate pattern follows a mathematical curve, can we follow that backwards to an estimated time of arrival?

I also know very little of the subjects involved, but I had heard something similar in regards to other things.  Please don't hurt me.
gpt
It is tricky to track this back purely using maths. It is generally true that the spread happens exponentially, however this only really applies after a certain threshold of cases. 

If for example the first people to be infected were people with extremely small social circles, or closed social circles (a group of people that has no contact with any person not belonging to that group, like a small isolated village for example) the spread would have happened much slower. Since we can not know what the social interactions of the first infected people looked like, there is no way to extrapolate the spread of the virus into the past purely using maths.

The way studies have found potential early infections is by detecting antibodies. Specifically there was a study done in Italy that tested blood donations made in 2019 for Covid19 specific antbodies and they have been detected in samples collected as early as september 2019.