Minutus

To quote Claude Bernard: "The microbe is nothing - the terrain is everything"

While I agree that viruses don't directly cause anything, they do invade normal cells and splice into the RNA/DNA strand thus producing a 'mutant.' There are also virus sized living entities known as mycoplasmas that are bacteria in miniature and these, along with viruses, are filterable meaning that they are small enough to pass through a ceramic filter media that won't pass yeasts, fungi or bacteria. Mycoplasmas are complete living entities and unlike viruses, they do not require a host to function.

We owe a great deal of gratitude to the late Lida Mattman Ph.D,(1912-2008) for her discovery of cell wall deficient forms known as 'L-Forms' as a result of her research. These entities hide from the immune system by living inside our red and white cells thus been given the appropriate name 'stealth pathogens.'

Everyone should know that standard light microscopes cannot see virus sized organisms which is why swabs are taken and then grown in the lab upon nutrient media. These tests are virtually worthless because the growth media shifts the sample pleomorphically and thus the media determines what the resulting bacteria, yeast or fungi will be. When so-called 'experts' pontificate about viruses they are clueless because they don't have adequate tools to detect them properly.

Electron microscopy is also useless because the samples are damaged and distorted by the process and worse, cannot be observed alive. Understanding the species can only be accomplished by observing them in a live state. Determining what a virus does in the order of things is impossible. It would be like determining the theme of a movie by only looking at the lobby posters.

Laboratory researchers are like children in the sandbox. Some dominate and others are obedient but one thing they all have in common, they discover and accomplish nothing and are paid large salaries to remain ignorant by looking into the light path of a brightfield microscope unable to observe anything buy large particles because they are too stupid to realize that they are blinded by the light. If one wants to really see what is going on in the true world of microbiology, they will equip them selves with phase contract or darkfield scopes. Caveat Emptor! Carmi Hazen

And hijack the cell's mechanism such that it stops doing its intended job and instead churns out more viruses, until the cell itself explodes -- we have that on electron micrograph. Isn't that pretty much of a direct "cause"...?
That's a "cause".
(Some have been fascinatingly useful, e.g. the one that is hypothesized as the reason why mammals are able to bear live young; most are either benign, or harmful.)