The rationale of life forms
The most common theist answer to the question of "why is there suffering in the world" (in this case, more specifically, "Why is there a parasitic catfish that swims up your private parts and eats you from the inside out?") is that we are to know pain in order to distinguish it from pleasure. We need to know the bad to discern from the good. We go through trials in this life so that we can overcome them and become stronger. So, God makes these parasitic catfish and allows them to crawl up your urinary tract and do all kinds of damage to your innards because in the long run something good happens.
What is the secular answer? That all forms of life evolved to what they are now through a long series of mutations. Each species is doing what it can to survive and perpetuate itself. Parasites evolved mechanisms for feeding off of larger organisms, usually small enough to avoid detection. They have survived because their parasitic nature has helped them to do so. Sometimes these parasites actually live in symbiosis with the organisms they inhabit, other times they cause the host to become ill and even die. But why do these catfish exist? Because they evolved into what they are now, because natural selection has favored the mechanisms that these organisms have developed. There is no further explanation necessary because there is no claim that these creatures were created by an intelligent being with a plan in mind for them.
How about mosquitos? Tics? Ringworms? Viruses? Poisonous snakes? All of these are dangerous and harmful. An explanation must be given for how they fit into God's plan, if one is to adopt the belief that they have been created by an intelligent being. But, with the world view of evolution, it is quite clear that they simply evolved that way. There are visible benefits to the mechanisms they have evolved. A mosquito drinking the blood of another organism makes sense because blood is always rich in nutrients, thus the mosquito is well-fed. Snakes evolved poison to kill or debilitate their prey and then consume it. They exist because what they do works. They are capable of surviving, and thus they survive. There is no need to come up with a plan that includes an explanation for why they are there.
We, as intelligent beings, are capable of finding purpose or meaning. Sometimes we find it where it is genuine and other times we may find it where it does not exist. We can assign purpose to all creatures. We can give explanations such as that given by theists. But, that does not mean that the purpose we assign is accurate, nor does it mean that the creature assigned that purpose is actually fulfilling that purpose. Yes, we may be able to contrast bad situations with good ones to better appreciate the good. However, does that mean that snakes were invented simply to allow us to do that? To me, this is absurd.
No, it seems much more logical to me to assume that these creatures exist simply because they have developed survival mechanisms that were superior to other species which no longer exist. We humans are the most prolific and populous species because we have developed superior intelligence, which I believe we have demonstrated far outweighs any other single mechanism ever evolved by any species. In fact, it is because it so greatly facilitates our ability to adapt. If the climate grows cold, we are capable of understanding that and wearing thicker clothing and building more insulating shelter, whereas an unintelligent species would not be able to do so. It would have to rely on physical changes in its own body, which may not happen fast enough to save the species from extinction.