Why did linking cure the virus in "What You Leave Behind" instead of transmitting it?

Question

In the final episode of Deep Space Nine, Odo links with the Female Changeling to cure her of the morphogenic virus that is killing the Great Link (Odo himself having been previously cured in the episode "Extreme Measures"). However, on previous occasions when a healthy changeling linked with an infected one, the virus was simply transmitted from the latter to the former, spreading the infection. Indeed, it wouldn't be a very effective virus if it could be cured simply by linking with a healthy changeling. So why was the Female Changeling cured instead of Odo simply being reinfected? Same applies to the end of the episode, where Odo

rejoins the Great Link, thereby curing them all.

Answer 1

You are looking at two states:

1. Healthy.
2. Infected.

You note that in every other example, the second state dominated the first.

But there are actually three states:

1. Healthy, never-infected.
2. Infected.
3. Healthy, cured.

In every other example, an infected shifter infected a healthy, never-infected shifter. I.e. the second state still dominates the first in this list. But Odo is in the third state. He was previously infected and is only healthy because he was cured. Apparently the cure persists after the disease is eliminated.

This behavior is consistent with how real diseases work. When a healthy person encounters someone with (for example) chicken pox, the healthy person can catch chicken pox. Once the sick person recovers, that person can neither transmit nor catch the disease again.

It is unsurprising then that Odo doesn't become sick again. He's still cured. It's also unsurprising that he can transmit the cure. Left indeterminate is what happens if a healthy shifter were to link with Odo. Would that shifter change from the first state to the third? Or stay in the first state? Either seems possible.

As you note, we are never told that the disease and cure work this way. But it is a reasonable way for it to work and is not contradicted by what we are told. Prior to the final episode, they could have had it work the other way. We only learn that it works this way from observation.

Answer 2

At this point we can only assume Odo carried a "vaccine" of sorts. By linking with the other Changeling he shared his immunity, or at least the ability to combat its effects. Similar to sharing thoughts and experiences.

Answer 3

[The cure] was a "Simple genetic marker" consisting of four nucleotides.

A genetic marker is a DNA sequence in a known location on a chromosome. We'll have to ignore the "four nucleotides" bit because, well, that part doesn't make sense.

The only obvious way that a genetic marker could be part of a cure is if the disease itself was deliberately engineered to self-destruct if it found the marker present in the genetic code of the cells or cell-analogues that it was attacking. We are talking about an engineered disease, so this makes sense; it would be reasonable for the designers to include such a failsafe.

The cure, then, would be implemented with something along the lines of a retrovirus, to insert the necessary marker into Odo's genetic code. Such an insertion would be permanent, and in a Solid would be inheritable; it seems reasonable to suppose that linking, since it involves two or more Changelings intermingling, would allow for an interchange of genetic material, which means that the genetic marker could be passed on in the same way.

So basically the fact that Odo could cure other Changelings by linking with them does more or less make sense - as much sense as one can reasonably expect from Space Opera, at any rate - given what little we do know about the disease and the cure.