So as far as multicellular virus go, I’m not sure. These ideas are using a vector organism that creates and injects viruses. I don’t see a way that viruses can create multicellular organisms without at some point being one.
My first idea would be a creature that employs the host cells to create masses of stem cells which then differentiate much like a regular eukaryote embryo.
The first stage would need to involve creating a shell or ‘womb’ to encase the alien cells, most likely presenting the surface proteins of the host cells.
After this the embryo would form itself inside the ‘womb’, protected from antibodies by the shell.
Multiple wombs would spring up in areas of the host with highly regenerative cells - for us unfortunately that’s the external membranes (tongue, mouth, gastrointestinal lining). If the ‘seeding’ virus bodies have a capsule to protect them from the stomach then they can reach the nutrient-filled and quickly dividing cell population of the gut, though there will be competition down there.
The main problems I see with this is mutation and evolution. Viruses are simple and quick at reproducing, so mutations are fast and so is adaption. They create so many ‘offspring’ so quickly that even if most mutations are unviable then the number of null and positive mutations still produces a net positive increase. With larger organisms this is harder. Still talking about mutation, creating mutation-prone masses of stem cells very quickly is just asking for a tumour.
Obviously this concept doesn’t exist in nature, but you can bet Earth is weird enough to get close.
Read this sh** and tell me it isn’t terrifying.
I know it’s an article so even though I recommend the read here’s the gist:
Stem crustaceans (basically a barnacle but not really) called Sacculina float around in a planktonic larval form, looking decently plankton-like until they land on a gap in a crab’s armour. They then insert a barb into the crab and inject the head-mass of cells called a vermigon into the blood stream and then to the intestine in which it then takes root - literally - and grows throughout the entirety of the internal tissue of the crab.
Once it’s got a hold, it gives the crab (regardless of sex) the ‘I’m pregnant protect the eggs’ hormones and forms a tumour-looking body that bursts through the crabs gonads (destroying them) and starts cooking up babies. The crab will clean, nurse and protect this sac as of it were it’s own eggsac.
Once a free-swimming male comes in to fertilise the crab climbs to a high place and releases ‘their babies’.
This is obviously only highjacking the reproduction of an animal in a physical way, as they still need a male parasite, but the lifecycle can be used as a basis. The virus slug injects their microscopic vermigon into the host, taking root and multiplying, but specifically using the host cells to do the cell production.
The problem with this example is that the single source and single eggsac means this is basically mass cloning, and for complex life this is usually not sustainable for a viable gene pool.
The Worst of Both Worlds
The slug infects either sex, and the virus injection quickly travels to the gonads, completely replacing the germ cells’ DNA. Once mating happens between an infected and a healthy individual (perhaps the virus promotes sexual drives) both will become infected. This first mating will be unsuccessful as the germ cells will not merge. Once a second mating occurs (this parasite may rely on second chances) both germ cell populations will have virus DNA and will successfully germinate. The female will then nurture the parasitic offspring (maybe multiple) and give birth to them. This one is not fun.
The advantage: technically sexual reproduction without ever creating their own germ cells. Complex life-viable gene diversity is sustained.