Ticks can transmit the bacteria that causes the debilitating Lyme disease at any stage of their life. See how small they are in the larva stage (the smallest tick in the pic), thats why a lot of times people don't even know they have been bitten.
And forget the old rule of no bullseye rash, no Lyme. While the bullseye is one of the many possible symptoms that the lyme disease bacterium Borrelia burgdorferi has been transmitted, it's not always going to appear. In fact, many times people don't get symptoms (or noticeable symptoms) until they are a few months in and start getting joint pain or worse, brain related issues.
This is why prevention is the key to avoiding this terrible disease. How many times, since my quest on learning about ticks and lyme, have I heard people say they, or their aunt, or cousin, or daughter had odd debilitating symptoms for years before they or the doctors clued in that it was Lyme disease. Unless you "get lucky" and end up with the clear tell tale signs, Lyme disease can easily go undiagnosed. And the longer is it undiagnosed the more difficult your life can become.