Novel about a parasite/symbiote that causes immortality

Perhaps Hyperion by Dan Simmons?

It's been a few decades since I read it, and I don't remember any airlocks, but there is a (cross-shaped) parasite called a cruciform which makes the host immortal.

After many weeks, Duré steals into their sacred cavern and discovers a finely wrought cross-shaped artwork that predates human spaceflight by millennia. He deduces that they are survivors from a seedship crash centuries earlier who have been infected with cross-shaped organisms called cruciforms that integrate themselves into their host. After death, the cruciform rebuilds the physical body and resurrects them. The price of immortality is that resurrection information is lost, and over time they become unintelligent and androgynous, losing all distinguishing features. When he is seen topless while bathing the Bikura discover he has no cruciform on him. Debating killing him they instead lead him into Hyperion's labyrinth system where he encounters the Shrike and is unknowingly infected with a cruciform.

Or indeed, could it be Neal Asher's story "The Engineer" about seriously deep bioengineering in the 1998 anthology The Engineer (reissued in 2006 as The Engineer ReConditioned)?

In it Asher's Polity-universe protagonists (a "partial cat-adapt" and her human male sidekick) discover the sporulated form of an extinct race of ancient bioengineers and attempt to revive it. This slow process leads to it biologically assimilating them and amplifying their abilities. Immortality? Well, they manage to survive the blast from a nearby nuke strike.

Sounds kinda like The Million Year Hunt, by Kenneth Bulmer. In it, the protagonist is inhabited by an alien symbiont and sets out to find the symbiont's mate/partner. Towards the end, both the protagonist and his girlfriend are thrown into space, and they only survive due to the symbionts' protection.

This list of summaries mentions the symbiont.

A friendly alien parasite occupies a human's body, giving him extraordinary powers in order to enlist his aid in tracking down a dangerous enemy.

Hal Clement's "Needle" has an alien symbiont fuse with an Earthling to find an alien criminal, and the alien gives powers to the host. The 1987 movie The Hidden and Star Trek's Trill race repeat the concept. The original Star Trek episodes "Metamorphosis" and "Is there in truth no beauty" repeat the concept.