Can an immigrant enter canada if they have an unknown illness?

As an immigrant, you must meet the requirements of the medical examination. This screens applicants who may be deemed a health or social care drain on their public system in Canada. So even if you met all the other requirements such as pass mark, security background checks, you would fail an application if deemed unsuccessful at the medical stage.

As a brief visitor, you won't need screening.

as long as it does not pose a danger to the public, (and as long as it is NOT contagious) and is NOT Tuberculosis then it should be OK.

The question of health does not usually come up for visitors unless you obviously dont look well.

probably need to have medical insurance