Aren’t organisms trying to find the most hosts so it can have more chances to survive ? sure it would kill all its hosts in short time but organisms don’t rationalize like we do, so why won’t it seek to become way more transmissible ?
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Evolution favors viruses that will keep its host alive long enough to pass the virus and not necessarily a particular method of transmission. Some viruses live in your mucus (airborne when you sneeze/cough), some in your blood, some in your genital mucosa, some locally on your skin (herpes/hpv).
Let’s ignore the anthropomorphic language here, and look at why HIV isn’t an airborne virus.
There’s lots of families of viruses, and they all tend to spread I’m different ways. Similarly, we’ve got different families of animals. Some fly, some walk, and some swim.
The viruses we’re most familiar with are the respiratory viruses because everyone gets those. You’ve got the flu and Covid, and all the viruses we just call a common cold. Those are flying viruses. They live and reproduce in the tissues that make up your respiratory system, so they spread by air. Their ancestors did it, and their ancestors’ ancestors did it.
Then you’ve got viruses that spread through blood. That’s where you got hepatitis, HIV, and a few STIs. Those are the swimming viruses. They like to stay in the blood, the lungs and nose just doesn’t do it for them. Their ancestors did it, and their ancestors’ ancestors did it.
Now viruses don’t really like to walk, but there’s a few bacteria that do. For the most part you’re only getting viruses the other two ways. You might cough up a virus and it’ll live outside for a bit, but it’s not their main groove.
Now we get to point where evolution doesn’t make giant jumps from swimming to flying. Given a few million generations it could happen, but even for viruses that’s a long time. All these families of viruses have had hundreds of thousands of years of evolution to get to where they are now, if not more. The common cold isn’t going to become an STI as much as HIV isn’t going to become airborne.
Certain viruses affect certain cells. In the case of HIV, it replicates by binding to the Tcell in the blood stream. T cell concentration elsewhere may not be high. Flu and Coronavirus affects certain cells in the lungs which makes it much easier to xmit by air.
Saying organisms like viruses are “trying to” or “seek to” do certain things is not accurate. As far as current scientific understanding goes, they do not have any kind of thoughts or planning that would allow them to “try” or “seek.” They have a single purpose in their existence, to spread and multiply, and they go through random mutations that cause them to change over time. The changes that lead to higher rates of spreading and multiplying will be more likely to stick around and continue to spread and multiply. This does not mean they will always move toward maximum infectiousness or maximum reproduction rate, only that they will naturally tend to move in these directions if there are no other things to consider (like being able to avoid or disable an infected host’s defenses).
HIV happens to have evolved in such a way that it can’t survive in the air or in saliva for very long, so it just can’t transmit well by those methods. It does however to a VERY effective job of reproducing and evading the defenses of our immune systems, which is why very few people have ever been fully cured of HIV. When it does infect someone, it sticks around for ages and can spread through sexual contact indefinitely, whereas other viruses like the flu eventually get beaten by the immune system and a person does not stay infected forever.
It’s all a matter of randomness and mutations. HIV is an effective virus (by which I mean it’s successful at spreading and multiplying) despite not being easily spread through air and saliva. There’s a possibility that some strain of HIV could become airborne, but that’s also just a matter of randomness and chance and probably taking a long long long time for many mutations to pile up that make it able to survive in those conditions long enough to infect people.
While the other answers here are all correct there is much more nuance as to why different viruses have different modes of transmission and the full explanation is much more complex than the “tall better than wide” strategy that others mostly talked about, it’s actually pretty hard to explain it simply and with Eli5 vocabulary so bare with me if this comment gets really long but I will do my best to make it as readable as possible to someone without the background in microbiology. To really make sure that you understand it I will start from a brief explanation of how viruses work in general.
Viruses are often described as “perfect parasites” – they are not classified as a living cause they lack the basic requirements of “life”: they don’t have metabolism, they don’t have any autonomous form of expressing or interpreting information and they follow the scheme of systemic assembly instead of organization (this two terms will be important later and I will elaborate more on what they mean then). Viruses are psychically made out of only two or three things: a protective barrier called a capsid that sometimes also has an additional layer of “camouflage” called a lipid membrane and the genetic information stored within the barrier called a virion core. Computer malware is a good analogy to how viruses infect us and why: let’s say that you download a photo from the internet that actually has a trojan virus inside, that photo is the virus’s capsid, the code inside is the core, then you use a photo editing program – so something with a specific purpose in your PC, analogous to a specific cell in our body – this program sees that photo only as a photo so it “opens” it and the malicious code inside gets transfer into the program, now the program becomes infected and the virus becomes half alive for some time, creating something called virion-cell hybrid, this hybrid then copies and inserts the malicious code into any further file that was created by that program, and sending that file to someone else is the same as virus infecting different cells.
Now with that background present let’s finally get to answering your question. Firstly, let’s start with the part where i called the hybrid “half alive” – even though virus practically gains control of the entire cell it doesn’t utilize enough of it to be considered living: it still doesn’t have a way of interpreting or sending information into environment and it doesn’t gain any metabolism – it never directly adds or receives energy from the host metabolic system, it affects it but changes in it only affect the host, not the virus itself. What it does however is that it forces the host to use it’s internal mechanisms of systemic self-organisation to use energy and substances that the host produces for itself to instead create the parts that them will self-assembly into viruses. Ok, I realize that you probably have no idea what the last sentence meant but let me correct that – in biology the term “system” refers to a collection of things that together create something different, for example your body is a system of organs, your organs are a system of tissues, your tissues are a system of cells etc. And systemic organisation and assembly are terms that define how the system is created and how it works – a simplified explanation is that systems that are organized exchange energy between the elements and ones that are assembled don’t, for example mitochondria are organized cause they exchange substances between one another – a rotaprotein won’t work without hydrogen from mitochondrial membrane, a virus is on the other hand assembled cause a capsid works as a capsid even without the virion and vice versa. Another important part is the creation and stabilization of the system – mitochondria use some of the produced energy to replicate and keep themselves stable, viruses have proteins that are magnetically charged on the end of the capsid walls, over time the walls created by the host bump into one another due to natural cytoplasm flows and they get stuck together assembling into a permanent capsid – it’s a entropic process that happens in energy equilibrium, nothing gets added or removed. That’s why they are called the perfect parasites – advanced parasites steal energy from the host but they can’t transform it 1 to 1 into offspring production cause some of it is used by them to “organize” them, viruses are entirely assembled so any energy they steal is only consumed by the creation of future virion parts. Now the way that a virus spreads and why it is limited if it is depends on how it’s evolutionary background interacts with the process off self-assembly. You see self-assembly is kind off like building a house without nails – if one element doesn’t fit, is a bit off or is lacking everything will fall apart – and so most viruses are fine tune over generations towards a specific host cell to ensure the fastest replication possible, cause being evolutionary tuned towards a specific host ensures that you can create as much of exactly needed virus parts as possible. Now let’s use the computer analogy to illustrate what happens if a virus infects through a different way than it evolved towards – we can say that it’s like opening a photo with trojan using mp3 player, the virus will attach it code to the mp3 player, but what now? Well there are 3 options – either the trojan is so simple that it will work universally in every program but then it will be also much easier for your antivirus to detect it – in those few cases the virus needs just the most basic from the basic organelle aparati to create it’s parts and those parts are so simple that they will quickly self-assemble – this evolutionary answer is a time game for the virus cause it sacrifices defensive mechanisms for speed – a beat the body before it beats me style that usually is the most common in the mild infections like flu, or it’s so complex that it has different approaches to their target program depending on what it does – it uses a different code for a photo or mp3 or anything else – this evolutionary answer is present mostly in “rare” viruses like viral liver infections – because you can’t be best at everything those viruses sacrifice adaptations to specific approach in order to be decent at all options – they aren’t the fastest nor the safest and usually they are connected with those viruses that stick out a long time in the environment – they happen rarely because their incubation speed is only faster than our organisms response if it never had an interaction with them before but they ensure to spread and multiply if this is our first time, and the last option is that it won’t work – simply it’s so specialized in infecting photos and in avoiding detection while doing so that it will give up on working with anything else – like HIV virus – over time it evolved on a specific host cell and as such it is adapted to every inner working of a specific cell that it can use them to much better advantage – HIV for example is able to use it’s host cells lim T to create a false protective cell membrane for the offspring and to do so in such a way that the offspring will self-assemble inside of it and it’s not that hard to imagine that if such a virus ended up in a cell that works slightly different it would quickly fall apart cause it won’t be able to control it precisely as it needs. Specializing for a host cell also makes the virus less resistant against threads that it would face in other environments than the way of spreading it adapted too, but getting better at one point of entry makes it easier for the virus to get in undetected – to summarize everything viruses that specialize are openly better at fighting our immune system due to being better at manipulating our cells,and even if they spread slower through the group cause of it they will survive longer and produce larger numbers in every single person.
And why those specialized viruses don’t evolve into different modes of transmission is a case of statistics: if a virus replicates in a specific cell it’s offspring will be released into a tissue with similar cells, now let’s say that two of that offspring are mutants – one is better at overtaking the same host cell and other trades some of that effectiveness to be more open on different options, now which one will overtake your organisms faster and as a result is more likely to be passed on others and who is at lower risk of detection? The one that is more efficient not the one who is more inclusive.
It has been found that [saliva rapidly disrupts almost all](https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/484948) of the cells that hold HIV – which makes the virus unable to infect someone new.
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