Re: elemental form - Seems alright, because it scales the type of elemental off your HD it improves nicely as you get higher levels. I would recommend rather than the wording about retaining personality and intelligence that the caster retains the mental ability scores of his/her original form; if you're concerned about them being able to cast spells in the new form, you could just have that specifically excepted, as with Tenser's transformation.
The material component is kind of weird; finding 100gp of precious stone is likely to be significantly easier than the other material components. I think that it would do just fine with non-costly material components, personally.

Now, I don't really like the idea of requiring a spell in order to get the component for another spell (it effectively increases the casting time for the first spell, and if that's all the second spell does, as is the case with suspend fire, it makes it useless in all other circumstances). That being said, here's what I've got for your challenge:

Sickness Bomb
Conjuration (Creation)
Level: Assassin 4, Cleric 5, Sor/Wiz 6
Components: V, S, M
Casting Time: 1 minute
Range: 0ft
Duration: Instantaneous
Saving Throw: None
Spell Resistance: No

This spell conjures a diminutive glass jar, stoppered with a cork and filled with a noxious and infectious gas. The jar can be thrown as a splash weapon, breaking on impact to release the gas. All creatures within a 5ft radius of the target (including the creature the jar was broken upon, if aimed at a specific individual) must make a Fortitude save against the normal spell DC or contract any one disease of your choice, chosen at the time the spell is cast from the list of diseases given for the contagion spell. You are not immune to the disease created by this spell, and must attempt a save normally if exposed.

Material Component: A bolus of mucus.

Patient Zero
Level: Cleric 8, Sor/Wiz 8
Components: V, S, M
Casting Time: 1 minute
Range: Personal
Target: You
Duration: 1 hour/level; see text

As you finish casting the spell, you crush the three sickness bombs in your hand and inhale deeply. For the duration of this spell, you act as a carrier for up to three different diseases (those contained within the sickness bombs). You suffer no ill effects from the diseases so 'contracted', but any creature who touches you or whom you touch, including attacks made by you or against you by natural weapons or unarmed strikes, must make a Fortitude save against this spell's save DC for each disease you are carrying, or contract that disease themselves. In addition, you can spend a full-round action to cough violently, spewing diseased spittle in a 30ft cone; all creatures in the cone must make the same saves to avoid contracting your disease(s).
Multiple castings of patient zero, even by different spellcasters or using differently-diseased sickness bombs, do not allow you to act as a carrier for any more than three diseases. This spell lasts for 1 hour per level, up to a maximum of 24 hours.

Challenge: A spell that makes you feel warm and fuzzy. (Either the character or the player, and either literally or figuratively.)