So, for 40k: Infestation, basically, an ancient starfaring race explored much of the galaxy, and a fledgling humanity now spreads out to explore the galaxy and discovers the ruins of the ancient civilisation: space stations, ships, colony outposts, all devoid of life. However, many of them seem to have been opened to vacuum, or to have been incinerated on the inside.
Then as humans get closer to the Core Worlds of the galaxy they discover what happened. The ancient civilisation had encountered a mutating virus on some planet. The virus spreads through physical contact - and has a chance of mutating the victim into a carnivorous monster. When the monsters slash, bite or spit upon people, the person has a chance of becoming infected. The person doesn't know if they are infected or not, without medical scans, and if they are infected, they will suddenly transform into a monster!
In the game, every turn after being struck - but not killed - by an alien, a human must roll dice to see if they transform into an alien themselves!
Each soldier, however, can administer a medical injection 'test' on another soldier, to see if the person is infected or not. The upside of the test is that if clear, the person doesn't have to roll anymore and can act as normal. The downside is that if the roll fails, it means that the medical injector detected the mutation in the soldier, and the medical substance kills the person instantly - stopping them from transforming into a killer monster. So, a good roll from a medical test stops the person from having to roll for infection, but a bad roll means the soldier is immediately lost - but he won't be transforming into a monster and attacking the team.
There are a couple types of aliens. I was inspired by the Genestealer expansion for the original Space Hulk, which had sort of half-alien / half-human figures. Basically, you can get a human soldier and glue a basic Tyranid head on it. One arm is a human arm with a weapon, the other arm is a Tyranid arm with claws. So, it's a kind of human/alien hybrid. Some people cut off alien tails, and glue them onto the people as well, or cut off one of the human figure's legs, and put an alien leg on him too. It just needs a single box of basic human troops, and a single box of any basic Tyranid troops, and a bunch of hybrids can be made.
So there are ravenous melee monster aliens, melee and weapon shooting hybrids, and then there are boss aliens, which use psychic powers to control the swarms.
With the mutation spreading among the human expeditionary forces who were exploring the galaxy, the military is now heading out to the Frontier to eradicate any traces of the mutation, and to rescue survivors who aren't yet infected (though each one must be tested to see if they are okay or not!). Otherwise, humanity may become extinct like the other ancient alien civilisation.