Caution: All canonical information regarding the wall sickness comes from Haibane Renmei ninth and eleventh episodes. The only fanfiction work known to extensively explore the wall sickness, its origins, causes, and treatments is FHD Remix. The author has triple-checked that wall sickness as presented in FHD Remix (and all related factors) is consistent with what little is depicted in the anime series. He has also determined after-the-fact that the apparent differences in the two series cases (Rakka and Hyouko) are consistent with FHD Remix wall sickness theory.
Wall sickness is the effects of poisoning by delta radiation, which is an unusual form of radiation better able to conduct through solid objects (including the Wall itself) than through liquid, air, or vacuum. It is more able to conduct through high density materials and fluids than low density materials. The most dangerous form of delta radiation poisoning is through contamination with a delta radiation emitter (or "vector"). Like other forms of radiation, emission is measured in Curies, and exposures measured in Sieverts.
The pattern of symptoms is consistent across all dose levels. First, the victim feels warmer and delta radiation effect acts as a painkiller both against the sickness itself and against any concurrent injuries (therapeudic use of delta radiation is a bad idea!) To others, the victim feels unnaturally cold (anti-fever or hypothermia.) The victim never dies during this phase, but depending on the dose, this phase may end sooner (high doses) or later (low doses), and may last from as little as two hours (10Sv) to three days (0.2Sv). After this, the victim experiences a fever which, if applicable, may persist even after death and reach very high levels. The victim may even vaporize ("I'm afraid to close my eyes. It feels as though I'll disappear if I go to sleep." - Rakka, Episode 9). This is a summary of all known cases of delta radiation poisoning after the Wall was made, dates are provided in SCD format (according to the calendar of FHD Remix):
Haibane Rakka (Haibane Renmei) SCD+1205 November, 2.5Sv: Exposure occurred at ground level from about two seconds of direct contact with the wall, Rakka had a minor sprain in her left ankle at this time. Anti-fever ended after about three hours: Treatment with conventional dextromorphan administered by Kana shortly after the fever broke out had no effect ("I gave her some, but the fever won't come down. I was going to take her to the doctor when the sun comes up". - Episode 10) Successfully treated with Bradford herbal remedy from DT4 (Haibane Renmei Temple) administered by Reki approximately 14hr after exposure.
Haibane Hyouko (Haibane Renmei) SCD+1200 September, 2.0Sv: (Caution: most of this has been extrapolated and may not be accurate): Hyouko and Haibane Reki undertook an unauthorized scaling operation with a ladder that was too short. After reaching the top of the ladder, Hyouko inserted a wedge into a crack to anchor the ladder, resulting in a long, low intensity exposure. Due to the delta radiation effect, the wedge froze, startling him into falling off the ladder. Shortly afterwards, ice in the crack ejected the wedge. The delta vector outside the wall is settled upon the ground, so exposure rates are lower if the wall is touched well above the ground. Reki was not exposed. Hyouko suffered some trauma to his head and extremities, but during the anti-fever phase, Reki was able to help him get to Old Home, and then sought treatment from the Haibane Renmei, which was successful in saving his life. Unfortunately, unauthorized scaling of the wall is a serious crime as it may compromise Glie's defenses. Both haibane were arrested, and the incident divided the Haibane communities of Old Home and Abandoned Factory for years afterwards.
Haibane Kuramori (Haibane Renmei) SCD+1199 March, possible exposure of 0.25Sv or less: Haibane Kuramori, on the advice of the Haibane Renmei, sought the ingredients for Sin-bound feather bleach in the Western Woods and collapsed while returning to Old Home. It is possible that she had suffered a minor exposure (less than what one would get from actually touching the wall.)
Haibane Tatakai (Three Worlds In One) SCD+1260 May, 8.0Sv: Tatakai touched the wall at ground level for four seconds while she slew a large Hogosho no Shigoku creature and its accompanying invasion force using the Light of War, as well as repairing severe damage to the wall itself. Her anti-fever was over in less than two hours. Haibane Shimoni and Kurai rushed to DT4 (Haibane Renmei Temple), in such a hurry that they rode a scooter all the way to the temple gate. Tatakai herself had remembered the Bradford herbal remedy from her previous life, where she had experienced a lesser dose and wrote it down for Kurai, who was familiar with the garden. Tatakai ran an extremely high fever, the water in her cooling bath rising to near 50degC (120degF) as the housemother (not the same one as in the series) and Haibane Menmo struggled to keep her from literally burning up. The treatment was administered about four hours after exposure, and she survived.
Human Patricia (Three Worlds In One) SCD+1257 December, 6.0Sv: A twelve year old girl rebelled against her parents and ran off to touch the wall. She was found and recovered after exposure, but died after about 18 hours. Two days after exposure, her body burst and vaporized.
Human Leroy (Three Worlds In One) SCD+1261 September, less than 4.0Sv, 0.1Cu: Leroy was possessed by Lilith to infiltrate Glie, and was therefore exposed to delta vector as well as touching the wall. Lilith, however, stabilized his body until he was arrested about eight hours after exposure. By this point, Light of Healing was available, but treatment with the Bradford herbal remedy was successful. Unfortunately, Leroy was to spend the rest of his life confined to DT4 as Lilith could not be removed until his death, as well as his treason.
Haibane Taka (Battle of Haibi) SCD+76 December 15, 150mCu: The most serious known case of delta radiation poisoning was taken by the de facto mayor of Haibi when she was attacked by an enemy wraith, who stabbed her in the leg. Despite valiant efforts by the Defenders and Western University Hospital (what Old Home was at the time), she died only 1.5hr after the attack through a combination of the effects of her anti-fever phase and conventional injuries. The Bradford herbal remedy was administered only eight minutes after the attack, and more advanced Katia factor treatment was given later at the hospital. Her wounds were treated with chitosan powder and bandages.