Biohazard

Starting Up
To begin, go to Ardougne and talk to Elena in the house just east of the one her parents live in. (Where you began the Plague City quest.) She'll tell you about how she was able to collect some good samples of the plague, but the Mourners took all of her equipment. You'll need to sneak back in to try to find it. She suggests 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. tells you he has arrangd 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 to West Ardougne. Use the pigeon feed on the fence (you can only reach one "square" of it) 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.

Infiltrating Mourner HQ
Head to the north-east corner of West Ardougne. You'll see a large building right up against the wall with a fenced in back yard. Nearby, you should see a rotten apple on the ground. Grab it. Now look at the fence around the back yard and you'll see a place where there's a part of the fence you can squeeze through. Use the apple on the cauldron and get back out.

Walk around to the front of the building, and try to get inside. The Mourner inside will tell you that everyone has suddenly come down with a bad case of food poisoning. Imagine that. Head south to where Sarah the nurse lives (near the altar). In the cupboard in her house you can take a uniform. Put it on and go back to the Mourner's house again. Now that you're dressed up as a medical professional, they'll let you inside.

Go upstairs and kill the Mourner (level 13) there. You'll get a key. Use it on the gate to get inside and search the crates to find Elena's distillator. Exit the Mourner HQ and head back to the south-east corner of the city (or exit via the sewers). Kilron will help you get over. Go back to Elena and give her the equipment back.

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. First you'll need to stop by the chemist in Rimmington to get some more touch paper. 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. Teleporting will shatter them. If you bank them first they'll be fine. If you do screw up, though, you can get more from Elena. You'll just have to go all the way back.

Your next stop is Rimmington. (You can teleport to Falador and pick up your items from the bank there.) You must have the vials with you to speak to the chemist. Once you get there, go to the chemist's house at the west side of town. Go inside and talk to him but do not mention the plague, or he'll take your items. Just say you're a friend of Elena's and that you need some touch paper. However, you won't be able to get to Guidor in Varrock with the samples you're carrying. There's a guard who will take them from you. Instead, you'll have to smuggle them inside. The chemist will suggest giving a vial to each of his three helpers, since you'll be better able to sneak them in if all you've got is one item. He does mention that each of his helpers have their own unique personality traits.

Give Chancy the liquid honey. He would sell the others, but liquid honey isn't worth enough for him. Give Da Vinci the ethenea, since he cannot use it to make any sort of artwork. To Hops, give the sulphuric broline, since he'll get thirsty and drink the other two.

Now go to Varrock (remember, don't teleport with the plague sample) and get your priest gown (both parts) (or some coins, since you can buy the gown in the costume shop). Head to the south east corner of the city to the gated area. (You may have noticed Chancy, Da Vinci, and Hops in The Blue Moon Inn, but do not talk to them here. If you do, they'll give you the vials back before you're safely through the gate. Then you'll have to go back to Rimmington and arrange to smuggle the items again. Instead, go right to the gated area and walk inside. The guard will search you, but won't find the plague sample. Now talk to the three helpers in The Dancing Donkey Inn to get the items back.

Put on the priest gown (since Guidor's wife doesn't want to let anyone in but a priest) and enter Guidor's room. Give him all the items, and he'll analyse them. He comes to a rather interesting conclusion, that there really isn't a plague, and that the mourners are hiding something. Go back to Elena and tell her what you found out, then take the information right to King Lathas in Ardougne Castle to complete the quest.

Rewards

 * 3 Quest points
 * 1,250 Thieving experience
 * Use of King Lathas's training ground