City of the Sun — Brian Stableford

The fourth landing of the Daedalus Mission confronts Alex and his companions with a colonial culture seemingly modeled on a classic Utopian dream, but all of its inhabitants are infected with a mysterious alien parasite, and they no longer seem entirely human. Are they being controlled by the parasite, or has the parasite merely enabled them to transform themselves? Can the visitors from Earth avoid infection themselves, and what will the consequences be if they cannot? For once, the risks of the contact seem potentially far greater than any possible reward--but that still leaves the visitors with the necessity of passing judgment and deciding what to do, in an exceedingly awkward situation. Daedalus Mission, Book Four.