Among the 6 secretion systems in bacteria, can these 6 ways be sorted out in the order of how harmful it is to the human host? Like say type 3 is highly virulent so that comes first, but I don't know about the rest of the systems. Being a computer science student, I have failed to understand the Wikipedia article to derive this order. Does a discrete order even exist? Can anyone answer this question?
Theoretically there are two very basic things to consider in judging virulence for the secretion systems: a) Can it secrete into any human cell? (If not, chances of being virulent are smaller, but not zero as whatever is secreted in the environment of a cell could also be harmful for the cell.) b) Does it secrete some agent (protein, RNA, molecule), which is potentially harmful for a human cell?
a) alone is not sufficient for it to be virulent if b) is not also given. (As it could inject something in a cell that the cell can handle without a problem.) b) alone is not sufficient, as it has to get inside the cell somehow. Although, b) alone can work, if the agent has another way to get inside a cell or to harm the cell from outside.
So, generally, without knowing ALL the bacterial secretion systems and secreted agents, it is impossible to decide if one secretion system (alone) is always, sometimes or never harmful.
I think you cannot find a better answer then the following review:
It was published in 2015, and it gives a good overview about the secretion systems. Anyway, T6SS can also be virulent. Vibrio cholerae can use an effector of this SS to cross-link actin in the host cell and modify its morphology and its cytoskeleton behaviour. In general I think that there is not a general order of virulence, also because the T3SS was discovered long ago. Whereas, the T6SS just recently, so the amount of information about its function is still somehow limited.
Hope this helps.