Different Germs. Two totally different germs. Erilichiosis is caused by a ricketsial bacteria lyme is caused by a borellia spirochete. They both may have very similar symptoms, can be spread by the same ticks, and have similar treatment. Erlichiosis does not cause the classic ecm bullseye lyme rash.
Tick borne disease. Both diseases are transmitted by different ticks and both ticks are infecting with different bacterium.