Biohazard

Official quest description: Now that Elena has been rescued, it's time to get to the bottom of this plague. Can you help her find out just what this mystery illness is?
Release date: 23 October 2002
Start: Talk to Elena in her house in East Ardougne. She lives right across the road from where you started Plague City.
Difficulty: *****
Length: Medium-Long
Members only: Yes
Requirements: Must have completed Plague City.
Items needed: Priest gown top and bottom
Monsters to kill: Level 13 Mourner
Spoiler warning: Plot and/or ending details follow.

Starting Up
The starting point of the Biohazard quest is in north-west East Ardougne
To begin, go to East Ardougne and talk to Elena in the house just east of the one her parents live in (where the Plague City quest begins). She will tell you about how she was able to collect some good samples of the plague, but unfortunately the Mourners took all of her equipment. You will need to sneak back in to try to find it. She suggests that you go talk to Jerico, since the tunnel you used to get in before has been sealed in.
Getting in
Jerico lives in a house just south of the northern bank in Ardougne. He tells you he has arranged for Omart to help you with a rope ladder. Search the cupboard nearby to find some pigeon feed and grab a pigeon cage from the back yard.
Now head to the gate in West Ardougne. Stand in the corner and use your birdfeed on the watchtower fence to throw it up all over the tower, then release the pigeons. They'll immediately fly into the tower to eat. This is just the distraction you need. Go south to Omart and talk to him. He'll help you get over the wall.
[edit] Infiltrating Mourner HQ
Head to the Rehnison's house from Plague City. There is a rotten apple around the east side of the house that you should pick up. Look at the fence to the east and you'll see a place where part of the fence can be squeezed through. Once on the other side and in the back yard of the Mourner HQ, use the rotten apple on the cauldron (use the right-click menu for this), and then navigate through the fence again.
Walk around to the front of the building, and try to get inside. You will be informed that several mourners are ill with food poisoning, and they are waiting for a doctor. Head to Nurse Sarah, who is southwest of the altar. Search the box in her house to obtain a doctor's gown. Put it on and go back to Mourner HQ again. Now that you're dressed up as a medical professional, they'll let you inside.
Go upstairs and kill a Mourner (level 13) while still wearing the doctor's gown. You'll get a key that will automatically be placed in your inventory. Use it on the gate to get inside, then search the crates to find Elena's distillator. Exit Mourner HQ and head back to the south-east corner of West Ardougne. Kilron will help you get back over into East Ardougne. Alternatively, you can also head through the sewers. Go back to Elena and give her the distillator.
Delivering the Samples
(Be extra careful in this section. Read each paragraph carefully, or you'll end up doing more walking than you have to.)
Elena will extract a sample of the plague, but the results of her test are strange. She wants you to take a plague sample to her mentor Guidor in Varrock to see what he makes of it. You will first need to stop by the chemist in the western-most building in Rimmington to get some more touch paper, however. She'll also give you some liquid honey, sulphuric broline, and ethenea, mentioning that the liquid honey is worthless, sulphuric broline is deadly poisonous, and ethenea is colourless. From here on, be very careful. These items are fragile. If they break, you will need to have them replaced by returning to Elena. Teleporting will break the plague sample, and combat will break the vials. You can put them in the bank for protection, but they must be in your inventory before talking to the chemist or Guidor.
Your first stop is Rimmington. If you put the vials in the bank first, you can teleport to Falador and pick them up from the bank there, or use the cabbage teleport with the Explorer's Ring 3 for the bank in Draynor. Once you get there, go to the chemist's house at the west side of town. Talk to him with the items in your inventory, but do not mention the plague or he'll take them away, which will force you to go back to Elena. Just say you're a friend of Elena and you need some touch paper for Guidor.
You won't be able to get to Guidor in Varrock with the samples you're carrying, as the guards will confiscate them. You will have to smuggle them into the city. The chemist will suggest giving a vial to each of his three helpers, since it is easier to get into the city if all you've got is one item. He will also mention that each of his helpers have their own unique personality traits.
Give Chancy the liquid honey. He will sell the others, but the liquid honey isn't worth enough to him. Give Da Vinci the ethenea since he can't use it to make any sort of artwork. Give the sulphuric broline to Hops as he will drink the other two. Or you can drop the vial outside the gate and use telekenetic grab to retrieve them. This works well for those of us who don't read thoroughly enough.
Note: If by some chance you give them the wrong vials (as has been done by a fair number of players), you can correct the mistake without returning to Elena. You must first go to the Dancing Donkey Inn in Varrock without any extra vials in your inventory and speak to each of the assistants there. Then with the extra vials, you must go back to the chemist's house in Rimmington and correct whatever mistake was made. Or you can just go back to Elena to get them replaced.
After giving the assistants the proper vials, go to Varrock. If you wish to teleport there, you must put the plague sample in the bank first (the closest one is in Falador). Get both parts of the priest gown from the bank if you have them, or buy the robes from the costume shop in Varrock for 10 coins if you don't have them. Head to the gated area of the city that is south of the eastern exit. Make sure you have the plague sample and the touch paper in your inventory, and attempt to pass through the gate. The guard will search you, but won't find the sample. On the other side, head to the Dancing Donkey Inn and talk to the chemist's assistants to get the vials back.
Put on the priest gown and head to Guidor's house in the south-east corner of the gated area. Guidor's wife will let you through as long as you are wearing the robes. Give Guidor the vials and sample, and he will analyse them. He comes to a rather interesting conclusion. Go back to Elena and tell her what you found out, then head to King Lathas in Ardougne Castle to complete the quest.