I have a theory about the origins and purpose of viruses. That's very simple, but very weird.
The viruses evolved by using living creatures to reproduce themselves. This appears not to be a symbiotic relationship since the affected beings do not gain anything but an infection. But I believe that this is not entirely true. The virus is a vector that we can use to infiltrate into other beings. Just imagine the virus as part of us, or a tool created by us, evolved within us. When we infect others the material that originated by our metabolic reactions is now inside another creature. Of course, the DNA/RNA of the viruses is not our own, but what if we can intentionally direct them to others? This is a very aggressive mean of communication, but that's exactly the point. Guess what, we unconsciously spreading viruses to the people we want. But it does not apply only to other people, it applies to ourselves too.
Maybe we do not manifest the disease, we just have the viruses but we can get along with them. When we want ourselves to manifest the disease, our immunological system is altered and the viruses inside us are activated. Just like some parts of our DNA, depending on the type of the cell, some segments can activate or not. Cancerous cells are infected by virus in most cases, that's an example of how dangerous they are. Nothing new here, but what I am suggesting is that we are responsible for the manifestation of certain diseases. How many times we become weak when we are unhappy, have a lot of complains, or have destructive thoughts? So, this desire for change yearns for chaos, destruction, renovation. Our minds can do amazing things for good and for bad.
The Virus Theory
Going deep into this theory is that this is also a mechanism that we use to project our intentions into others. Since our thoughts cannot interact with the thoughts of another, by strongly manifesting intentions into different types of viruses this is a way to materialize our will to reach the other. I believe that our minds can do much at cellular level. Our will have much power not only to control our physics. It goes deep into our life system. Our will intrinsically connecting our DNA, and our DNA interacting with viruses RNA/DNA and its reproduction, mutation, etc. Life have thousands of years of existence, and all this system I am describing used the small chances and opportunities that opens up possibilities to take advantage of the best architecture that can be designed.
Exchanging DNA/RNA is not new in nature. Bacterias do it with their cross over method. It is easier for them since they live within the same environment. Multi cellular beings cannot cross over their DNA information since their cells cannot physically touch each other all the time. So virus is for multi cellular beings what plasmids are for bacterias. Evolved plasmids finding a way to make sense in a different form. Thousand years of evolution made this mechanism turn into diseases, but that's what see now. I believe that this is just one aspect that we perceive. The roots were good for the evolution. And even now, in a different or same way, we just don't realize it.
What is yet to be discovered about us, about our DNA, about life is unimaginable. Viruses mutates inside us and so our cells. What is inside our bodies is the environment that defines us. We are not just our cells, we are the mithocondrias, the bacterial flora living symbiotically and other aliens, every life form within the body. Its just natural that the lives in ourselves desires to survive, mutate and manifest into other environment or bodies. And its natural that the beings in us attempts to interact with each other, to every way they can. Usually the virus has a very specific target. Human viruses usually doesn't affect turtle viruses, etc. This is because we are not turtles so we don't have the necessity to project our DNA or our mutated DNA into turtles and vice-versa. There are multiple factors why virus are specie specific, I am just listing this one as an example.
Who knows all the mysteries of what can't be seen? The unknown can always be wondered and travel around like virus carrying information. Some are useful, some are just stupid.