The fevers do wear off over a while - they should fully recover after roughly 17 days of in-game time, with the worst symptoms hitting around the third or fourth day. That is, of course, assuming they aren't exposed again during that time...
Alchemy disasters can produce a variety of ailments, the most visible of which is radiation sickness (which can include light to heavy vomiting, fever, and very occasionally, excruciating death). Other possible results include (but are not limited to) your dwarf lighting themselves on fire (I lit up an entire map by accident one time, very FUN), temporary blindness, and easily-infected chemical burns and rashes. Though the possibility of a serious disaster is fairly low, alchemists who continue to experiment for long careers are much more likely to die or be horribly disfigured than just about anyone else, except perhaps a front line military grunt.
The next update of Dwarf Fortress is greatly expanding the possibilities for syndromes, so expect future developments in this department of risk and reward, particularly with the planned elements of the ritual tech tree. Version 0.14a will lay the tier one and tier two framework for this, but the ritual tech tree will not see a good portion of its features until 0.15, which is waiting for the next version of DF.
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A little more about the ritual tree. The ritual tech tree is another system of linked buildings that runs parallel to the mechanical tech tree. It is not necessary to being the mechanical tech tree to begin or advance through much of the ritual tech tree, though there are a few planned "crossover" elements that will require certain advances in both trees.
The mechanical tech tree, thus far, has been based on percentage-chance research and development, relying on discoveries and the like to progress. The ritual tech tree, though chance is not completely absent, is based much more on the investment of large quantities of resources. Initially, the only ritual buildings available will be the altars. While altars do contain a number of chance-based reactions (making offerings to certain spheres in hope of rewards and not retribution), these reactions are not the means by which the tree is advanced. Unlocking the next advances in the ritual tech tree generally requires the accumulation of enough resources to build a larger and more extravagant ritual platform. Sometimes, you will need a few of a rare resource (i.e. platinum) to construct the statues necessary for a new temple. Sometimes, you will need a vast quantity of mundane resources (i.e. stone blocks) to create the arches and spires of a new shrine. In many cases, you will need some combination of the two. As the dwarves "advance" down the ritual tech tree, the temples and altars will become exponentially more lavish and complex, and the rituals performed at each will grow potentially more rewarding.
Also unlike the mechanical tech tree, the ritual tech tree does not "unlock" structures in the same way. While dwarves can record and document their technical advances for replication, temples and shrines do not become significantly cheaper with time or advancement. Thus, unlike the mechanical tech tree's "back up" copies of advancement items, you will be more inclined to defend your ritual buildings, as constructing a replacement is much more of a daunting challenge. The sheer size (in terms of both resources and tile space) of later ritual buildings makes this increasingly difficult, but increasingly rewarding.
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While much of the ritual tree is still in the conceptual phase, a working preview of tiers one and two will be included with 0.14a.
Rest assured, the planned features for 0.14a's ritual tech tree are dynamic and very fun, but they account for less than half of what is planned in the future just for tiers one and two alone.