YSK that in the United States, only a small fraction of Lyme disease patients present a bullseye rash.
Why YSK: Most people know that the "bullseye rash" is the number one indicator of Lyme disease. However, only about 80% of patients even present a rash, and only about 20% of those present a bullseye appearance. In Europe, a bullseye pattern is much more common. This is due to the different strains of the bacteria present in ticks that cause Lyme disease.
In media, it's often repeated that Lyme CANNOT be the diagnosis if the bullseye rash is not present, but this is false. The most dangerous thing about Lyme disease is that people often don't realize they've been bitten by a tick, so it gets under diagnosed. If you've been bitten by a tick, monitor the site for a rash, but keep in mind that rashes don't always present. Pay attention to other symptoms as well.
https://tools.cdc.gov/podcasts/media/pdf/EID_08_21_LymeDiseaseUSeurope.pdf