Unless the galactic cross-section of life is weirdly small, I doubt that "an alien virus" would be a threat.
Our own viruses work with DNA (or RNA) codes that co-evolved with the living things they infect. There's no reason to believe an alien version of a virus is suited to infect a human being (let alone be worse than a random 'new' Earth virus). The chances it holds 'replicate me!' code compatible with our own cells is small, not least because it probably won't have the same codon-expectations, or even A(T/U)CG forming the codons, or even the same type of bases at all, or even on the same type of backbone, or even based upon the same carbon-chain chemistry, or
even based upon carbon! (To be honest, it is
thought that carbon is the only practical basis for life[1], but every stage further back towards the terran ideal adds progressively more chance of divergence.)
Predators, maybe. They could kill, even if they found us organically unpallettable (which they probably would) and then you'd just be relying upon instinct (or threat response), comparative scale, comparative
numbers of predator to prey (us), whether the alienness of our appearence pushes them towards caution or into "attacking attack"/"defending attack" behaviour... The first blob-eating-blob that tries to absorb a human through attaching to the sole of a space-traveller's boot is not going to have much luck (unless it's an extreme-pH 'attack' and the boot isn't proof to that). Perhaps more problematic could be inadvertently-transported herds of sauropod-equivalent grazers that don't even eat 'meat' but are evolved to root around house-sized rocks for nutrients and now rampage around damaging house-sized
houses desperately looking for something edible.
Long-dead civilisations are interesting. Probably nothing deliberately left behind, as time would have disabled/prematurely activated any actual ingenious unattended traps. But possibly their main ultra-long-term-radiological-waste repository is marked with alien symbology that means "no, seriously, don't excavate anything that is here" but
looks to human eyes more like "Congratulations you are the 1,000,000th visitor to this site! Dig here to claim your unique prize!"
'Sleeping' alien empires have much same issue. Obviously good enough to be hibernating/semipermanently in torpor (perhaps arranging a caretaker role for one or more individuals, by turns, to keep the rest of their stasis-pods 'stasis'ing) and capable of understanding our encapsulated-FTL system, but never worked it out for themselves before we stumbled upon them and triggered their wake-up call.
And, in all these, the reverse of the threat is true. If alien biology can infect us, we probably have something to infect alien biology. We may discover a planet with interesting bubblegum-flavoured land-jellyfish and end up doing what we did to the dodo or very nearly did with all the giant tortoises (and very actually did with some of them!) - or, if not us, our mice, rats, cats, etc that we accidentally dispersed there. Oh, hello Long Dead Civilisation... Did you leave a magnificent record of your great but ultimately futile works..? ...oh, I'm not sure, we inadvertently destroyed the key (lock-type, and/or Rosetta Stone) you thought you'd left for those who came after and maybe never even realised it. Or perhaps we discovered a strange energy source, brought in engineers to remove it for study, and now the Sleeping Chamber Catacombs are filled with the smell of (whatever your version is of) rotting flesh as your intended long-period sleep instead becomes terminally eternal.
(A lot of human-vs-alien fiction likes to treat us as the midway-point on social scales: neither as utopian nor as distipian as other represented societies; having far more dullard ones and others nigh-on intellectually superior; there's a more extreme warrior-race and another who rule as ultra-pacifist gods; or various mixtures from various ends of various mutually-exclusive scales. The humans, arriving on the scene, are the JOATs, in a way that no existing galactic race seems to have become. Together with some quality that seems to have burnt out the most amongst the Ancient Incumbents to the galactic plane, like tenacity or adaptability, we end up having compassion
and self-centred determination. We can learn to build upon found technology but then also be imaginatively destructive with it. Our verdency can be great but also
can be consciously restrained, meaning we always have people to explore/raid/hold-the-line/etc without plunging into mindless assimilation or becoming an entirely spent force. … But there's nothing to say that, in reality, we
are as seemingly one-dimensional as Klingons, Vorlons, Predators, nomadic-space-'Gods'... A race of Kal Els or Darkseids or Thanosii or Galactuses or even Tribbles, from the comparative viewpoint of all other races who would rather we just keep to ourselves. Maybe our microbes
are the only ones that don't live in perfectly symbiotic harmony with the other branches in our own tree-of-life. Or maybe symbiosis is that One Neet Trick that only our own planet has developed in its various gut-biomes, coral reefs, lichen blooms and we actually have the closest thing to the concept of Gaia. But in leiu of more than one planetary datapoint, our own, our very imagination remains utterly biased - and we have no idea which way or how much!)
I don't think it's true to say I Have An Opinion on this. I have
many possible Opinions, and still think it's likely that I'm missing the True Opinion, and at the very least am holding multiple mutually-exclusive ones open so long as I'm not proven that they are Wrong.
(PPE: Oh look. Partially, yet succinctly, ninjaed. Max hadn't posted that before I started this. All credit to her for using fewer words to say that bit better.)
[1] In any environment that isn't too far from our comfort zones. Extreme pressures/temperatures might support a direct silicaceous version of organic chemistry (with many other elemental tweaks, almost certainly, very unlikely to be an *ahem* Carbon Copy in all other regards). And who knows what 'life' can develop in stellar plasmas based upon small-scale(?) magnetic and electrical field interaction and coherent self-reinforcement. But none of this type of ecosystem would readily provide anything we'd think of as a 'space flu', or would be amongst the least concerns for the human who ventured to expose themselves thus.