"The Ark": TARDIS Attribute question

[vargaplant]

I'm in the process of watching some early episodes for the first time. Last night, I saw "The Ark", featuring featuring the infamous Monoids.

[MILD SPOILERS BELOW]

The Doctor's new companion, Dodo Chaplet, has a cold. She unwittingly infects the Guardians and the Monoids aboard the Ark. As both species lack resistance to this particular virus, it causes widespread illness and several deaths.

So why doesn't this happen more often? In the real world, the early European explorers brought smallpox to the New World, which decimated most of the native populations across the continent. You would think that the Doctor's travels would cause species-destroying plagues on a regular basis!

I haven't seen all (or even most, at this point) of the classic stories, but some quick research leads me to believe that this does not happen. If that's so, why?

Which leads me to my TARDIS atttribute question: does the TARDIS somehow disinfect its passengers? Is this a subsystem, similar to the one that facilitates language translation? And was it malfunctioning during the events of "The Ark"?

I've only skimmed The Time Traveler's Companion so far, so I don't know if anything like this is covered.

[Marnal]

There's a 2nd Doctor episode where the Doctor is asked about something like this an he says (IIRC) "The TARDIS is perfectly sterile."

The novels and audios go further and explicitly establish that the TARDIS normally prevents diseases and viruses from being passed from one time zone to another.

[Rel Fexive]

Perhaps that incident persuaded the Doctor to fix the faulty decontamination field and from that point on it wasn't a problem?

In other words, it only appeared as a plot point once and has been explained away by later writers.

[Marnal]

Sounds good to me!