This post is not offered as a guide to plague prophylaxis.  Most of the best ways of preventing or containing plague (like decent food and physician access) have been thoroughly explored in earlier posts to to this forum, and are best found through a search on the keyword "plague".  One of my favorites was Brugle's suggestion offered for plague-prone scenarios in reply 2 to "Itjawy how to avoid the plague" for setting up a couple of sacrificial huts with no health care in an isolated location. Instead, this post offers a few insights into the behavior of plague carriers for ambulomancers and other connoisseurs of walker behavior.

For walker researchers, plague carriers (also known as plague walkers) are a rare and valuable resource.  I recently captured one in Heh and, after many reloads and road tweaks, got him to reappear on another starting square.  Here are a few of the rules of thumb about plague carriers that I was able to glean from observing these guys and comparingtheir behavior to information provided by Brugle in reply 15 to "Walkers and teleporters from pavilions".

1.  A plague carrier appears at the beginning of the month.  However, his appearance seems to have a random component.  If you reload a game saved a couple of weeks before the carrier appears, there is no guarantee that he will appear when the calender clicks over to the new month during the replay.  (Obeys one of Brugle's rules about carriers.)

2.  The plague carrier seems to originate from the house (usually a hut) with the worst disease risk on the map, as indicated by the "Z" overlay.  (Obeys another of Brugle's rules)

3.  The plague carrier appears on whatever roadsquare has the highest priority in the table below, where H stands for his house (1x1) of origin and lower number denote higher priority.  In the table, north lies towards upper left.
 
 

Fans of roaming-walker behavior will recognize that these carrier-appearance squares follow the same order of priorities as the walk-finish squares around 1x1 buildings (squares 1 to 8) or around a 3x3 building centered on top of the hut (priorities 9 to 24).  (See Brugle's "Random walker start and finish points" for details.)  They also follow the same order as the search sequence used by the algorithm to hunt for roads around a routing center during roaming-walk generation as described in "Ambulomancy: Predicting Roaming Walks in Pharaoh".  The use of the walk-finish pattern also confirms one of Brugle's rules.

4.  The carrier's square of appearance is determined shortly before the pop-up "PLAGUE!" announcement appears on the screen.  Once the announcement appears, it is too late to delete the road on the carrier's chosen square of appearance to persuade him to appear on the next highest priority road.  He will simply appear on the newly vacant square and walk to the nearest road.  Even dropping a small statue on his chosen square of appearance (after deleting the road) will not deter him.  He will materialize within the statue and march out of it towards the nearest road.

5.  The carrier's path (at least to the first dead end) is determined after the pop-up "PLAGUE!" announcement.  Cutting roads before the carrier appears forces him to turn around at a newly created dead end.  In contrast, he will march right across a road cut made after he appears.  (So pause, drop speed to minimum, and save the game as fast as possible if you ever see a  "PLAGUE!" announcement.  You can still try different ways of controlling the carrier.)  This way, you can make two surgical road cuts and trap the carrier on a tiny, two-square piece of road.

6.  After the carrier appears, you can force a recalculation of his path by cutting the road in front of him and dropping a small statue in the gap.  Although the carrier is prepared to materialize within a statue (cf.,  rule 4 above), he is not prepared to march into one.  Once again, you can trap him on a small piece of road between two statues.

7.  A house become infected as soon as the plague carrier comes within two squares of it.

8.  The plague carrier does not need to return to his house of origin before disappearing.

9.  In my experience, the plague carrier's walk always has length of 33 squares.  Counting his square of appearance as square "zero", he will vanish on square 33..  I know of no roaming walker in Pharaoh whose walk length never changes.
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10.  The walk of plague carrier does not appear to be made with reference to a customary walk target.  This property also is unlike all roaming walkers.  He certainly has no fixed destination, so he could not possibly be considered a "destination walker".

11.  The plague carrier is not in ordinary, "random mode" (See "Ambulomancy:...") for a default walk of non-standard length.  For one thing, he passes through roadblocks freely.  See also the next rule.

12.  A carrier whose walk-start square falls on a road corner does not exhibit predictable directional preferences, and he certainly does not follow the same directional preferences that roaming walkers display when starting their default walks.  I have started carriers in different west corners.  Sometimes they go southeast; sometimes they go northeast.

13.  A carrier whose walk-start square falls on a straight road does not always head "East", like default walkers do.  On a NE-to-SE road, my walker headed SW, although he did head SE on a straight NW-to-SE road.  I have not had the (monumental!) patience needed to create enough carriers to see if they actually have a reliable direction preference when starting in a straight piece of road, but I would be pleasantly surprised if they did.

Thus, the plague carrier appears to be a special case, handled by the algorithm like no other walker.  He cannot be assigned to any of the usual three endurance classes (short, medium, or long walkers).  He does not use walk targets.  He does not make classical default walks.  Since the carrier from each diseased house only appears once (before the house gets effectively reinitialized) we cannot tell if he would quadramble or not, but since the four-walk cycle is implemented by rotating through the routing centers I do not see how he could (since he ignores them).  I cannot make myself call a plague carrier a "roaming walker", but he certainly is not a destination walker, either.

The only good things about the plague carrier is that he can be contained, and people who don't do something stupid (like I did by keeping a single hut from evolving and eating by letting it get too close to a fort) usually do not need to worry about seeing him - unless of course they are in Itjtawy.

StephAmon