make a virus that infects everybody but basically harmlessly. It's just that only in the presence of certain exact genes does it turn deadly.
This is Dunning-Kruger on stilts.
It's a sci-fi trope being confused for actual understanding of an insanely complex issue, and I am going to stick my neck out and say that you have no education in either biochemistry or molecular biology, and as a result are blissfully unaware of how incredibly hand-wavy that "just" actually is.
How, exactly, do you arrange for a virus to "turn deadly", and how do you propose to use the mere presence of certain genes to achieve this?
Bear in mind that the majority of harm done to people infected with harmful viruses is actually a consequence of the immune system's response, and that viruses themselves are tiny - they carry almost no information in their genomes other than that for making copies of themselves, and even for that purpose, they mostly co-opt the host's equipment, as coded for by the host's genome.
Further, it's worth remembering that the genetic code isn't much like computer code. There's no language as such; Just chemistry.
It's not something you can hack in the way that a programmer can hack code to do almost anything for which it has sufficient permissions and/or access; You can't execute arbitrary code in a human cell, though you can tell the cellular machinery to make an arbitrary sequence of amino acids into a protein. Guessing what effects (if any) will result from building a given primary protein sequence is essentially impossible, and is three stages of vast complexity away from the quaternary structure, that (if you could miraculously make it to your hoped for design) might or might not catalyse or suppress the reaction(s) you're interested in, which in turn might (or might not) have the effect you're hoping for in a particular tissue, which yet again in turn may or may not lead to the medical symptoms you wanted.
Three and a half billion years of evolution have made today's complex organisms, including humans, incredibly resistant to viral attacks, and viruses have been in an arms race with immune systems for as long as immune systems have existed.
Any "engineered from scratch" viruses are therefore likely to be complete failures, and bioweapons are basically just modified versions of naturally occurring pathogens (or in some cases, just the unmodified wild-type pathogens collected and then placed in an environment where they are usually not found, or not expected). This severely limits what can be engineered; If you wanted to build a helicopter, but were limited to using only unmodified parts from motorcycles, you'd struggle to produce anything effective - and that analogy seriously understates the problems with building a bespoke and controllable dangerous virus using only existing natural viruses as your starting point.
It makes a great basis for a cyberpunk novel; But it's no more realistic as a proposal in the real world than saying "Build a spacecraft like the shuttle, and
just add a system that can fold spacetime, so you can travel to the stars in less time than the oxygen takes to run out".
Easy to imagine. Not easy to do.
