The practical implications of what follows will already be known to anyone who's read any of the Rabies guides in the sticky, but hopefully it should still be of interest. This is a summary of an investigation posted last year in the Technical Discussion forum of the Amazon Basin (where I have the same forum name as here).
A rabid bite from a Werewolf does two things: it applies Rabies' poison damage rate and the Rabies state, both for the displayed length. The Rabies state in turn applies two things to its target: the rabiesplague overlay and missile. When a target is described as a host, infected or infectious, it means that the Rabies state has been applied to it.
Once the Rabies state has been applied to a target, it cannot be overwritten i.e. if a target is bitten with level 14 Rabies and then bitten again with level 21 Rabies, the state will still last for the remaining length of level 14 Rabies.
The rabiesplague overlay is the graphic displayed when a target is infected i.e. the orange 'aura'. The invisible rabiesplague missile is attached to a target for the length of the Rabies state: it fires one rabiescontagion sub-missile in a random horizontal direction every 4 frames, each of which travels for 20 frames before expiring. This is why Rabies can result in lag and being dropped from the game when many monsters are infected (although this isn't something I've experienced in single-player).
Rabiescontagion can (and does) hit the host but also pierces, so a single missile can hit multiple targets if they are all within range. Rabiescontagion is auto-hit and cannot be blocked by monsters.
When a target is hit by rabiescontagion, it applies Rabies' poison damage rate and the Rabies state, both for the remainder of the initial bite's displayed length i.e. if a target is hit by rabiescontagion one second after the original host was bitten, both the rate and the state will be applied to that target for one second less than the displayed length.
In this way the infection spreads: the Rabies state will expire simultaneously on all targets whose infection was a result of the same initial bite.
Interaction with Other Poison Sources
The poison applied by Rabies abides by the same rule as all other poison sources: a poison damage rate (poison damage per unit of time (frame or second)) that is equal or greater will overwrite any previous poison, applying its equal or greater rate for its length instead. However, the mechanics of Rabies result in certain... peculiarities.
While Rabies applies its poison and state with a weapon, the poison from items and Venom also applied by that weapon does not combine with that of Rabies; this is unlike Poison Dagger and the javelins of Poison and Plague Javelins.
Instead, Rabies is treated as a separate and thus competing source; if it has a lower poison damage rate, Rabies will not overwrite another poison source. However, if the other poison source has a shorter length than Rabies, poison can be re-applied by rabiescontagion once the other poison has run its course.
Furthermore, Rabies applies poison multiple times to its targets; as mentioned earlier, once the Rabies state is applied (whether by a bite or rabiescontagion), rabiesplague fires rabiescontagion every 4 frames, and rabiescontagion can hit the host as well as other targets within range. This explains why Rabies is the only poison skill that can kill rather than simply reducing an opponent to one hit point when duelling: even if the most recent rabiescontagion applies a lower rate, a frame of that rate will still be applied each time. This also has implications for maximising Rabies damage.
An infected monster's own rabiescontagion missiles apply a frame of poison damage with each collision instead of any current poison: however, rabiescontagion missiles from other monsters apply a frame of poison damage as well as any current poison, which can result in higher damage with infected mobs. If a single target is infected, it won't take additional frames of poison damage.
Level 1 Rabies has a length of 4 seconds (100 frames) and increases by 0.4 seconds (10 frames) with each additional skill level; the poison damage rate of Rabies also increases as the skill level does. Aside from placing points in its synergy (Poison Creeper), Rabies' poison damage rate can also be enhanced by the following modifiers:
- Lower Resist Curse
- +% to Poison Skill Damage
- -% to Enemy Poison Resistance
All three of these modifiers are applied before poison damage rates are compared, and only need to be active when the poison is applied. Lower Resist applies Poison Resist -%, reducing a target's poison resistance by the displayed percentage; +% PSD enhances poison damage by the displayed percentage, and is multiplicative with the synergy bonus; and -% EPR effectively applies PR -% (to targets that aren't Immune to Poison) and Poison Length Reduced by -%.
Since monsters normally have no PLR % (the only exceptions are Ancients whose magic equipment spawns with PLR % suffixes) this means that -% EPR increases poison length by the displayed percentage. -100+% EPR thus doubles poison length (both PR -% and PLR -% are capped at -100%).
Since Rabies applies variable poison damage rates multiple times with a diminishing length each time, -% EPR can result in varying poison lengths. While the Rabies state spread from a single bite will always end simultaneously, targets will remain poisoned for varying lengths depending on when equal or higher rates were applied.
Ideally, you (but preferably a Necromancer) would first cast Lower Resist; however, you have to shape-shift to use LR charges (making you more vulnerable), and the only % Chance to Cast modifiers are either too low level or awkward and unreliable.
Rabies' skill level is set when it is first applied, so changing it by switching weapons after biting won't affect the poison damage rate being applied i.e. applying level 21 Rabies and switching weapons to reduce it to level 14 (and vice versa) will have no effect. However, note that although the Rabies state cannot be overwritten, the poison can be; this means that if a target is bitten by level 14 Rabies and then level 21 Rabies, the higher rate (and accompanying length) of level 21 Rabies will overwrite that of level 14 Rabies.
Since Rabies' poison can be applied multiple times and skill level is set when first applied, Rabies' damage can be maximised by applying the maximum skill level possible (Plague Bearer Rune Sword's +5 to Rabies (Druid Only) and a Spirit 'TalThulOrtAmn' shield's +2 to All Skills apply the maximum +7) and then switching to a weapon and shield with +% PSD and -% EPR to enhance the poison damage rate and length as much as possible. These modifiers will result in the poison applied by the next Rabies contagion being greater and thus cause it to overwrite the previous poison.
Rate Versus Length
The longer you apply Rabies with the same modifiers, the more likely the rate will increase: subsequent applications could have a higher rate that will overwrite previous ones (at worst, the last will have a rate equal to the first). However, since -% EPR increases rate and length, the longer you apply Rabies with the same modifiers, the more likely the length will decrease: subsequent applications could have a higher rate that will overwrite previous lower rates, and in doing so will apply a shorter poison length. The choice is between doing more damage per second over a shorter length, or less over a longer one.
However, the variation in damage rate is only due to the difference between minimum and maximum rate at level 1; both minimum and maximum rate increase by identical amounts as skill level increases, so the damage range becomes increasingly small as skill level and total damage increase. For example, a fully synergised level 57 Rabies applies up to 61,287-62,047 Poison Damage Over 26.4 Seconds, a range of just 760; the minimum total damage is approximately 98.8% of the maximum. This means maintaining +% PSD and -% EPR modifiers for the entire length would only improve the damage rate by approximately 1.2% at best.
Thus to maximise total Rabies damage, you would cast at the highest skill level possible, then switch to the combination of +% PSD and -% EPR that maximises rate and length, then switch back again once you're certain those modifiers have been applied (this ensures that the poison isn't overwritten by one with an equal or slightly higher rate (but potentially much shorter length).