My cleric would eventually like to enter the prestige class cancer mage (Book of Vile Darkness 52-4). Once he's a cancer mage, what are the best diseases for him to catch both to increase his personal power and to debilitate his foes?
First, ask the DM how disease host works
Exactly what diseases a creature with at least one level in the prestige class cancer mage (Book of Vile Darkness (Oct. 2002) 52-4) should attempt to locate then catch depends a great deal on how the DM reads the cancer mage's level 1 extraordinary ability disease host, which says
At 1st level, a cancer mage suffers no ill effects of diseases, except for purely cosmetic ones such as boils, pockmarks, watery eyes, blackened skin, hair loss, foul smell, and so on. The cancer mage becomes a carrier of every disease he encounters, though he remains immune to most of their effects.… (53)
The traditional reading of the special ability disease host holds that ill effects means only the bad stuff (and I pick what's bad) rather than, for example, all of the disease's noncosmetic effects. This reading allows the mage to pick only the (noncosmetic) beneficial parts of, for example, the diseases festering anger (BV 30), vile rigidity (BV 32) and warp touch (BV 30, 31).1,2
However, while there's no official 3.5 revision of the prestige class cancer mage, Exemplars of Evil (Sept. 2007) includes the NPC Edgar Tolstoff, an otherwise unexceptional male human ranger 3/duskblade 2/cancer mage 2 that possesses the special ability disease host. Exemplars describes the special ability disease host as follows:
Edgar never takes damage from disease, though he might manifest cosmetic effects. Whenever he encounters a disease, he becomes a carrier and can transmit the disease to others. If Edgar is the target of a remove disease spell, he takes 1d6 points of damage per caster level and loses his blindsight for 1d10 days. (51)
A disease has a damage entry, and that entry is typically the entirety of the disease's effects, good and bad, so this version of the special ability disease host means the cancer mage ignores all of that (noncosmetic) damage even if that damage includes getting a natural armor bonus or something. This version of disease host makes a cancer mage playable even in a campaign featuring diseases that can deal beneficial so-called damage.
In addition to deciding which version of the special ability disease host to use, a prospective cancer mage should find out how the DM is reading that word encounters. The traditional reading holds that encounters means simple exposure is sufficient for the cancer mage to carry the disease forevermore unless circumstance conspire against him (e.g. a well-meaning (or enemy!) cleric casts upon him the 3rd-level Clr spell remove disease [conj] (PH 271)). However, a DM may rule, for example, that the cancer mage still must catch then keep diseases he encounters, making the cancer mage's existence much more challenging.
Finally, the DM may rule that neither of these versions is accurate and, instead, that the special ability disease works the way the DM wants it to so as to bring the special ability in line with the 3.5 revision (see Why a Revision? on DMG 4).
Second, ask the DM how the mage spreads diseases
Diseases spread by contact and ingestion are easily weaponized—touch a foe or spit in the foe's food. Many diseases spread by inhalation and injury are less clear. If the DM rules that a cancer mage that strikes its foe with a sword transmits the cancer mage's carried injury-spread diseases, that's gold, but if the DM rules otherwise, depending on the difficulty of spreading them, it may be that the cancer mage shouldn't even bother collecting injury-spread diseases. This is sad. (Also see here.)
Then get infected
If the DM's using the traditional reading of the special ability disease host, the cancer mage's priority becomes the aforementioned festering anger, vile rigidity, and warp touch. Using the Exemplars's version of the special ability disease host instead makes only the disease warp touch a priority as the cancer mage's diseases can only be used offensively.
The 3rd-level Sor/Wiz spell contagion [necro] (PH 213) can give the cancer mage the diseases blinding sickness, cackle fever, filth fever, mindfire, red ache, the shakes, and slimy doom (all DMG 292-3). These aren't particularly desirable diseases, but they're a good starting arsenal and usually convenient.
This player recommends the cancer mage's disease criteria be A) Does the disease have an incubation period of less than 1 day? and B) Is the disease's damage interesting? A great disease answers Yes to both questions. In the list below, supernatural diseases are italicized.
- The contact and inhaled disease ashdoom (Sinister Spire 18) (DC 14) has an incubation period of 1 round and deals 1d4 points of Constitution damage. Further, each time that saving throw is failed, a second saving throw is made immediately, and failure means that the ability damage becomes ability drain. Rather than dying, a victim that has an ability score drained to 0 by the disease ashdoom gains the template plaguelost (19), becoming an incorporeal and, if not already, an undead creature. Interestingly, the disease specifically affects corporeal undead creatures' Charisma instead of a living creature's Constitution. The disease is specific to this adventure.
- The injury disease beetle rot (Sharn: City of Towers 182, 183) (DC 16) has an incubation period of 1 min. and deals 1d6 points of Constitution damage. It's spread by the rancid beetle swarm and the rancid beetle zombie (a CR 6 swarm of undead creatures and a CR 6 undead creature, respectively).
- The contact disease a fell magic disease (Dragons of Eberron 138) (DC 20) has an incubation period of 1 min. and deals 1d4 points of Dexterity damage and causes blindness as the "[a]ffected creatures painfully sprout vines from their skin and eyes; any whose Dexterity is reduced to 0 transforms into a clump of vines." The disease is carried by living vines in Xen'drik's Abandoned Orrey (137-8),
- The injury disease gaki fever (Oriental Adventures 163) (DC 20) has an incubation period of 2 hours and deals 1d6 points of Constitution damage. It's carried by the shikki-gaki (a CR 3 undead creature).
- The injury disease hunefer rot (Epic Level Handbook 199) (DC 35) has an incubation period of instantaneous and deals 1d6 points of Constitution damage. The saving throw, though, isn't repeated daily but repeated each round. A creature that dies from hunefer rot shortly thereafter "forms an 18 HD mummy… under the hunefer’s command," the Epic Level Handbook's author seeming to assume a hunefer (a CR 25 undead creature) caused the creature's death, not an ambitious cancer mage. (In other words, ask the DM how this works.)
- The mysterious disease Kyuss's gift (Elder Evils 143) (DC 25) has an incubation period of instantaneous and deals 1d6 points of Constitution damage and 1d4 points of Wisdom damage. The disease is unique to the EL 20–21 encounter "The Obelisk" (142-3), so contracting it's challenging.
- The inhaled disease lungrot (Underdark 110) (DC 20) has an incubation period of 1d4 hours and deals 1d4 points of Strength damage and 1d3 points of Constitution damage. While mundane, only magic cures it. It "strikes air-breathing creatures that have been sealed inside ancient ruins," so you might have it already. Depending on how the DM rules the mage spreads inhaled diseases, a mage can enter a town, wait a week, kill it, and take its stuff.
- The contact disease "a particularly virulent strain of the disease called gray wasting" (Manual of the Planes 110) (DC 20) has an incubation time of instantaneous and deals a shocking 3d6+6 points of Charisma drain! Becoming infected with the disease requires taking a seat on Siege Malicious, the throne of the oinolth while the oinoloth—the ruler of Oinos, the first gloom of Hades—remains undefeated. Look! An adventure! (If the oinoloth is defeated first, then having a seat on the throne doesn't infect the creature with the disease, but other cool disease-related abilities are gained. Remember the old cancer mage saying: "Grab a seat then defeat!")
- The inhaled disease pit lung (Fiendish Codex I 112-3) (DC 16) has an incubation period of 1 day and deals 1 point of Wisdom drain. Unimpressive as that seems, 1d6 days after being infected, the victim's alignment becomes chaotic evil (or, as cancer mages call it, chaotic awesome). Encountering the disease is difficult: it's present only "[i]n places where the physical cohesion of the Abyss crumbles" causing "a cloying cloud of oily gas [to issue] from the surface of the plane itself." Good luck.
- The contact disease plague (Monster Manual III 125) (DC 21) has an incubation period of 1 min. and deals 2d4 points of Dexterity damage and 2d4 points of Constitution damage. It's spread by the plague spewer (a CR 10 undead creature), and a Huge o bigger creature that dies from the disease plague becomes a free-range plague spewer. Have fun.
The injury disease slaad fever (MM 230) (DC 18) has an incubation period of 1 day and deals 1d3 points of Dexterity damage and 1d3 points of Charisma damage. Further, a creature that has its Charisma reduced to 0 by slaad fever becomes a red slaad (228-9) or, if the creature can cast arcane spells, a green slaad (230). The blue slaad carries slaad fever.
As can be seen by other entries on this list, there're several diseases that transform a creature into an undead creature superior to the pedestrian ghoul fever (MM 118-9), but this disease is among the only that transforms a creature into something not undead.3
- The injury disease spiritual corruption (Sharn: City of Towers 178-9) (DC 13) has an incubation period of 1 round and deals 1d4 points of Wisdom damage. The disease is carried by swarms of feral spirits (a CR 6 undead creature).
- The injury disease Stygian wasting (Draconomicon 188) has a saving throw DC equal to the Styx dragon's breath weapon's saving throw DC, hence from 13 to 37. It's incubation period is 1 day, it deals 1d6 points of Charisma damage, and it takes three successful saving throws in a row rather than two to cure. Carried by the Styx dragon, the creature's extraplanar subtype makes it a target for spells like the 5th-level Sor/Wiz spell lesser planar binding [conj] (PH 261-2) et al.
- The injury disease zombie plague (City of Stormreach 144) (DC 16) has an incubation period of 1 day and the victim "loses 1d6 points of Constitution" (presumably this is ability damage but ask the DM). A victim need make only one successful saving throw to recover from the disease, but a victim that dies from zombie plague arises as a zombie that spreads the disease zombie plague! The disease is carried by zombies animated with the 5th-level Sor/Wiz spell animate infectious zombie [necro] (ibid.)
This list omits mummy rot (MM 190-1) (and, similarly, the greater mummy's mummy scourge (Deities and Demigods 160) and the bloodrot's blood fever (Heroes of Horror 140-1)) as well as lycanthropy (MM 170-9), both of which this DM considers more curses than diseases, but ask a DM about mummy rot et al. specifically because they're great. Also, don't forget to ask the DM if the cancer mage can carry afflictions (Book of Exalted Deeds 34-6), that are like diseases but only infect evil creatures.4
1 The disease festering anger doesn't have a typical infection method. Instead, the disease infects the victim after the victim experiences "long-term, intense fury and hatred" for "usually… at least a year." This makes DM fiat pretty much the only way to actually catch it.
2 "Once [the disease warp touch] takes hold, no more saving throws are needed: The malady gets neither better nor worse," but it's unclear whether a creature can opt to make saving throws to recover from it so the creature can suffer from it again. Even using the Exemplars version of the ability disease host, the disease warp touch is among the cancer mage's best offensive diseases, requiring only contact then the creature making a Fortitude saving throw (DC 20); failure means that the infected creature rolls on an old-school chart where, for example, there's a 10% chance that the infected creature "turns to formless jelly [and] dies."
3 Everyone talks about the shadowpocalypse and the wightpocalypse, but no one ever talks about the slaadipocalypse, which is so much more giant frog.
4 The DM will probably say No.
Festering anger is traditional:
Which is why the answer to “Is the Cancer Mage overpowered?” is very much yes.
Vile rigidity has similar mechanics, and can grant a cancer mage arbitrarily-high natural armor bonus to AC. Scintillating scales can then turn that into a much-superior deflection bonus to AC.
Both of those diseases are right there in Book of Vile Darkness alongside the cancer mage, which is just mind-boggling really. Scintillating scales is from Spell Compendium.
Outside those two diseases, however, there’s little and less worth being a cancer mage for. Diseases rarely have upsides, tend to be weak as attacks, have low DCs, and immunity to them is fairly common anyway.