This is generally true, however there is always a residue of uncertainty. If the Martian organisms were functionally similar to Earthly lifeforms there would be an admittedly remote possibility of sufficient 'overlap' in metabolic processes to allow them to produce disease.
Even functionally dissimilar organisms could not be automatically assumed to be 'safe'. Although the likelihood is vanishingly small, there would still be a possibility that an utterly alien life process could produce the biological equivalent of 'Unintended Consequences' if Earthly life was exposed to it.
Assuming that cross-contamination of organic materials (and even primitive life forms) has occurred between Earth and Mars the possibility of finding at least primitive functionally similar life there increases.
Fortunately, the odds would also increase that Earth's and Mars' 'cousin' life forms would be at least partially adapted to one another, and that both groups would have the ability to muster at least some degree of immune response to the other.
Utterly alien life, on the other hand, would have the (admittedly remote) chance of possessing, purely by chance, a chemistry that is in some unpredictable manner reactive with Earthly life. The reaction might easily be equally destructive of the alien life causing it but that would be little comfort to life on Earth if contact with that alien life were to precipitate a self-sustaining reaction. Since there would be no shared biological ‘history’ between the antagonistic life processes, there would be no moderating effect of natural selection to cap lethality at sustainable levels. Such a plague could explode with geometric rates of infection and literally sterilize an entire biosphere in a very short period of time.
In the absence of comprehensive methods to simulate the incredibly complex interactions that might occur between radically different life forms the only ‘sure’ method would be to expose a cross-section of the human population to the alien life form and then observe what happens.
Certain classes of condemned criminals might be allowed to volunteer for such an experiment, and earn remission of their punishment by providing this service to mankind. Communists might also be used in situations where volunteers were not forthcoming when and where needed.
Of course, in cases where communists were used, it would only be natural for the researchers to be hoping for a little ‘bio-drama’ to unfold on the other side of that foot thick porthole...
History is replete with people too timid to explore - we call their decendants Europeans. :)