Viruses need to be inside a living thing (of course the living thing has to have the right temperature which most living things do) . When they're inside a living thing they start injecting the host's cells with its DNA in order to make more copies of itself which allows the virus to spread. So basically viruses need to be inside something living.
I do believe that it is both naturally occurring and caused by human activity. Although science has proven that it is something that naturally progresses with time, humans have sped up the process. Hope this helps:)
i believe a stem cell is the only type of cell that can grow into any cell. meaning it can go anywhere it may be needed.
The fibrous proteins which are found in connective tissues of the animals are either collagen, elastic, or reticular fibers. This collagen fibers provide strengths to the connective tissues.
Hope this helps