The theory is that because we're so much cleaner now, our immune systems are not maturing and developing normal immunity, leaving us vulnerable to more and more allergies. We tend to inherit the tendency to get allergies from our parents. So, if you have allergies, you can blame it on them. You can be allergic to one thing or many. It depends on the individual and what you're exposed to.
Most people are allergic to things they inhale, like pollens, molds, pet dander, dust mites.
Anything you can inhale, you can become allergic to. No difference in symptoms, just in frequency. Don't have a pet. Some people wash their pets, though I don't know how you would wash a cat every week because it has to be done at least that frequently. But if you can get your cat or your dog washed on a consistent basis, you can wash enough of the allergen off that it's not as much of a problem.