Insects are the vectors as each one carries with it a small community of other organisms.
Some are merely phoretic passengers, but others live more or less intimately in or outside their hosts. In the high mountains of New Guinea, large flightless weevils live amidst mosses in humid forests.
The backs of the long-lived, slow-moving weevils are camouflaged with living fungi, algae, lichens, and liverworts. The integument surface and setae, plus a secretion, encourage plant growth. Mites, nematodes, rotifers, diatoms, and poked insects live and feed among the plants, causing no harm to the weevils.
Several microbes and occasional helminth worms are usually associated with all other insects. Lower forms of life were present before insects evolved.
We can speculate that as scavengers in decaying vegetable matter the early insects would have been immersed in a microbe-rich environment and would have ingested and excreted saprophytic forms. Inside the insect’s gut, some were digested and absorbed, and some persisted harmlessly or as injurious parasites.
In time, some microbes evolved a beneficial partnership such that their hosts could exploit foods that were otherwise nutritionally deficient. The mobility of insects and their resistance to desiccation allow their less hardy associates to survive and be dispersed in the terrestrial environment. When insects began feeding oh the vascular fluids of higher plants and vertebrates, certain viruses and microbes were introduced into the wounds.
Some proved to be highly dangerous to their new hosts. Insects also became involved as intermediaries in the natural cycles of other disease agents. Although insects alone rarely cause death of plants and vertebrates, as vectors of disease they can have catastrophic effects on entire populations.