[giarc]

Almost all microorganisms as suspectible to UV light. The damage happens at the DNA level (UV light causes a thymine dimer when two thymine nuclobases are found together in DNA... thymine being the T in the AGCT of DNA code). This dimer can result in frameshift mutations which can result in a bunch of different damage depending on where the TT bases are (some mutations result in no change, some result in cells inability to replicate etc).

So that's the background on how the UV light acts and disrupts a cells ability to survive. Now, about your question on how could a virus evolve if UV light kills it. Easy... the virus doesn't spend a ton of time outside of your body. Most transmission is person-to-person, there isn't a ton of person-to-surface-to-person spread therefore it's not like the virus is sitting on a handrail in the sun for days. Also, as I mentioned above, some mutations don't result in any change. There is a lot of redundancy in the DNA code (see DNA codon table 1). Not every combination produces a unique aminoacid.

1 - https://en.wikipedia.org/wiki/DNA_codon_table

My microbiology background is a bit dated, so my facts might be a bit off here and there.

[brlewis]

> Most transmission is person-to-person, there isn't a ton of person-to-surface-to-person spread

So I've heard. Doesn't this mean that the UV news is of little impact?

[giarc]

Yes, somewhat. I was involved in a project a few years ago where we installed specialized light housing in the ceiling at the hospital I worked at. These units had a UV system above it. Basically it would circulate air over UV lights and therefore would help to decrease the amount of virus floating in a room. So potentially this could help with a system like that but that only solves a small portion of person-to-person transmission. Much of it still happens with-in family groups where you are sitting and talking/hugging/kissing etc. That's why mask wearing and physical distancing are key.

[eloff]

Plus it's a bat virus originally, and bats are largely nocturnal, and also roost in dark areas like caves during the day. Resistance to UV light is not that useful to a bat pathogen.

[zeta0134]

Also it's really important to conceptualize the sheer scale at which a virus operates. One single virus isn't a problem, but when it enters the host's body and replicates, you're talking about millions of the little things. Some of them will die off to all sorts of natural causes and that's (from the virus's point of view) not a problem so long as enough of them survive to continue replicating, in their host, in some adjacent host, etc.

[supernova87a]

I guess this is the science behind the statement "sunlight is the best disinfectant" through the ages.

[giarc]

Correct. Since UV light acts at the molecular level and at the very core of what makes the cell work (DNA or RNA) it's hard for organisms to overcome. There are mechanisms of fixing these dimers, but cells need to produce them (which uses resources). Therefore there is a trade off, create too many fixes and something else has to give, create too little and a little UV light kills you.