I have now played the Mutation Challenge variant of Pandemic: On the Brink. This is the one that introduces the purple disease. Now instead of four, you need to cure five diseases. And this fifth purple disease behaves in strange ways. Also there are 12 purple cubes only, not 24, which means you can easily run out of them and lose the game.
With the Mutation Challenge, there are three special cards shuffled into the player deck, and two special cards that will be shuffled into the infection deck after the game starts. All these cards cause purple cubes to appear or grow on the board. Most tell you to draw one or more cards from the bottom of the infection deck, and place one or more purple cubes on the city or cities depicted. These cards are the only way purple cubes appear. Purple cubes grow via a similar way as normal cubes. If you draw a card from the infection deck showing a city that already has one or more purple cubes, you add one more cube.
The appearance of purple cubes is very unpredictable, because cards are drawn from the bottom of the infection deck. In Pandemic, the discard pile of the infection deck (which tells you which cities will have disease growth) is often shuffled and added to the top of the infection deck, so you will have some idea which cards will keep coming up again and again. With the purple disease, things get trickier.
To cure the purple cube, you use 5 cards of any colour, but one of them must be of a city that currently has at least one purple cube. This can be very easy if you happen to have the right card, but if you are unlucky, this can be quite tough, especially if you have already used or discarded many of those required cards. By spending cards to cure the purple disease, you will also have fewer to cure the other 4 diseases. So you need to be careful about using and wasting cards.
Michelle and I played the Mutation Challenge at medium difficulty. It took us three attempts to beat the game, compared to four with the Virulent Strain variant. The purple disease was very unpredictable, and it was sometimes tough to decide whether to try to eradicate it or to just contain it. The purple disease distracted us from other diseases, and also seemed to slow down the pace of the game somewhat. I'm not sure what gave me such a feeling. It may be because we kept being instructed to draw cards from the bottom of the infection deck, which meant more and more cards were moved to the top of the deck, thus "diluting" the infection cards that we would see. So disease growth seemed to be less intensive, although it seemed to be wider.
The Mutation Challenge is very different from the Virulent Strain variant. It seems to be slightly easier (admittedly based on very few plays). From just these few initial plays, I like this less than the Virulent Strain variant, because the experience is less "fast and furious". Not that it's easy, just that how it kills you is different. The Virulent Strain stabs you in the heart. The Mutation Challenge bleeds you to death from multiple cuts. How's that for visual shock?
Having played a few more games with the various new roles, I find that these new roles are very good value for money. I discovered many interesting uses for some of the abilities of the new roles. Many of them are a lot of fun. For example, the Archivist can pick up a card from the discard deck. One trick is you play a card to fly to a city, then take the card back immediately, and use it to build a research lab, all on the same turn. The new special action cards are fun too. One of them allows you to switch roles. This can be very very useful, e.g. after a disease is cured, you can switch to become the Medic, who free-treats any cured disease by just passing by.
Now the only part of Pandemic: On the Brink I have not yet tried is the Bio-Terrorist variant. Let's hope I'll get to try this soon.