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imagine multiple of logs flowing down the stream, they flow and flow depending on the current and surroundings, some of them break down eventually into smaller parts and at some weak points they accumulate, and then more and then more until river gets stuck and overflows flooding the area. then if multiple guys come and are able to clean up the river it might go back to original route again. see how stick never used actual energy but the process of stacking up in the spot was making the river overflow was just part of outside energies driving it. viruses are adapted in similar way.
example: you cough on a buddy and give the virus energy to move around in space, the buddy's mucuous system cells use energy to engulf virus because coincidentally(or not) it has exact molecules that these cells recognize and bind to, from then the cells use their own energy to produce viral particles because once inside cell can't tell difference between alien and own genetic material. once cell produces enough viruses that it can no longer hold it just bursts open and projectiles the viruses into the surrounding area, still viruses propell in space without using the energy and just follow the forces like movement of extracellular fluid or intravascular blood. repeating the process in as many cells as it can (because of compatibility) before immunity take care of it(or not). see now how the scenario is similar to the one above? this also will shed light on concept of why most viral infections are self limiting. viruses simply go with the flow. of course this flows can be manipulated by us through the use of energy(again) both for good and bad to human race.
