Health experts are still trying to identify what's behind a bug invasion that's left hundreds of people with itchy, red welts. It's usually just a nuisance, but ABC7's Kevin Roy found out Tuesday the bites can be serious.

When ABC7 first reported this story Monday night, doctors said there were no other symptoms, except for the aggravating itching that comes from those ugly red spots. But ABC7 talked to one man Tuesday whose case was so severe he was hospitalized and says he has never been that sick before.

"Chills, terrible chills, where I had eight blankets on, and then I'd throw them all off and spike up to 102-103 temperature," said Tim Ryan, bite victim.

Tim Ryan's symptoms sound an awful lot like West Nile: fever, nausea, fatigue, vomiting. But he says the spots on his body are not mosquito bites.

Ryan now believes the pimple-like bumps, with a 2-to-3 inch red rash around them, are from the oak leaf itch mite. There are two big oak trees right next to his backyard in LaGrange. On Saturday, his symptoms were so severe he checked into the hospital.

"I got real, real sick...nauseated, headache, fatigue, joint pain," Ryan said.

State health officials are still trying to figure out if the itch mite is the culprit. They have set up traps, including in Larry Prystalski's backyard in LaGrange, and in several counties, but mites were not identified in samples collected Tuesday.

"This whole block is infested, everybody's complaining about the bites and the itches," said Prystalski.

While the itch mite first appeared in Kansas and Nebraska in 2004, health officials don't know why it's showing up in the Chicago area now.

"It may have to do with weather and other environmental conditions, but we really don't know why there is an outbreak now and there wasn't one last year," said Dr. Linda Murray, Cook County Public Health Department.

Dr. Murray says only a very small percentage of people will experience symptoms as severe as Tim Ryan.

And dogs may have itching fits from it as well. At the Hinsdale Animal Hospital Tuesday, they could hardly keep up with the complaints.

"We've had probably 15 calls today alone from clients that are concerned. Does my dog have it, do I have it, does my dog give it to me?" said Dr. Frank Arnold, Hinsdale Animal Hospital.

Dr. Arnold says if your dog is scratching a lot try an oral antihistamine like benadryl for relief. A lot of humans have been snatching up benadryl and anti-itch creams -- so much so we're hearing that some drug stores are actually out of those products.

For most people, the bites should clear up on their own in about a week.