anorlunda said:
I read that viruses outside the host remain infectious only for a short time; hours or days. What happens to them? How are they damaged?
Most viruses live [sic!] in water, and I have no idea how longlived they are. It could depend on how easy it is for them to travel from host to host.
On land, destruction seems to be the case. Flu viruses are airborne, and most such viruses (flu, ebola) lasts for about 15 minutes. I would assume the sensitive mechanism they depend on to infect the host, which is usually fueled by immense internal pressures to inject the virus genome out of the virus capsid into cells, is destroyed.
Those destroying mechanisms could be either moist (flu virus doesn't transmit well in winter, and I have read this as a hypothesis why) or absence of moist (i.e. maybe longer lived viruses dry out and lose that inner tension). Unfortunately, seeing how vital it could be for combating virus disease, I don't think this is well researched.
anorlunda said:
A second related question. All living organisms require an external source of energy to survive (correct?) The words live and die are hard to apply to viruses, but do viruses need external energy to "survive" ?
"Life" is seldom a simple or useful definition.
More generally, "life" could be the process of evolution (the process of all life), and then its objects are populations rather than individuals. Under such a definition, viruses are life, while the last isolated sexual animal could be "residual life" or "non-life" or "rather extinct" (no partners).
If you use "life" as a means to identify individuals, you get something like the NASA definition for astrobiological search: an evolving, metabolizing organism. E.g. cellular organisms. And yes, such life need energy to metabolize. In fact, it may be the other way around, it can be free energy that once organized metabolism. (See "battery" or "alkaline hydrothermal emergence" theories for emergence of life.)
Secretly viruses needs energy too. They are parasites on cells, and use their metabolism for procreation. The best way to look at a virus is perhaps that the viroid (encapsulated virus) is the infectious inert spore, and the "molecular organism" that injects/insert and inhabits an infected cell is the living, metabolizing, adult parasite stage. It even works out if you see the inner pressure injection mechanism of phages as metabolic derived potential energy.
Some viruses form more or less cell-like "viral factories" instead of distributed "molecular organisms" inside the infected cell. Since some, or perhaps all, viruses seems to originate as parasitic cellular organisms* that as parasites can undergo simplification of "body plan" but complexification of "life cycle", that shouldn't surprise us.
*Even the capsid proteins hint at this. They root in cellular membrane protein pore complexes, exactly what you would expect to see kept but modified in parasites that insert (break down cell walls with excreted muralytic enzymes) or inject (use something like similar bacterial pore constructs) stuff into cells.
