Considering they are part of Godās creation they are good.
Very good was reserved for humankind in the creation story.
I believe it was St. Augustine who taught that all the organisms that are harmful to us werenāt meant to be so, but only to the lower life forms. Only after the fall did we become vulnerable to diseases, parasites, insects, etc. They serve a purpose in the natural world, such as population control of species, adaptations of species, ridding the environment of carcasses, etc. So, they are good, but not necessarily good for us humans. After all, there are good bacteria and bugs that inhabit our bodies, too. They must be there for a reason and since God created everything we have to accept that the reason is a good one.