Please help put my mind at ease!
If you are allergic to something, certainly bring your meds.
No, the place isn't crawling with all of the above, and in general not a big concern. I'd say you're more likely to get a bad sunburn. But if there are products you use that help with the mosquitoes, bring them as just having them may make you feel better. You'll probably forget to put them on after a day or two as you'll see there isn't much of a need. That's not something I ever think to pack.
I don't think you need to worry, but yet kharma will probably dictate that you'll be tested by something because of your fear.
If you can be mentally prepared to see a spider and disregard it (or know how to deal with it), you'll be better off than continually fearing you'll see one. I say that as I had a friend who visited me that was afraid of spiders, and I didn't realize the extent of his fear until he saw one.
He survived, of course, his reaction because of his fear seemed more dangerous than the spider itself. We now laugh at the experience, of course, but I know it's not a laughing matter. Spider bites themselves are rare.
Also, depending on the extent of your fear, you could probably buy a mosquito netting to sleep with. (
I in no way think this is necessary, especially in a resort area, but it may help you sleep.) They are still relatively common in many campos--so it shouldn't be impossible to find one.
And lastly, enjoy your trip. It's a beautiful country, and you've made a great choice to visit!
Focus on all the fun things you'll do and see.