When the Nivens visit the Titanian ship, they come across a tank full of humans who were alive. What were the slugs using them for?
The humans were test subjects who had a disease dangerous to the slugs (the true threat) which had taken over the Titanians (the slugs current host/carrier) called Nine Day Fever.
The Titanians were using them in an effort to find a cure for the disease which kills the slugs faster than their hosts.
This gives the book's protagonists, Sam, Mary and the Old Man the idea of spreading the disease among the human populace to stop the invasion from spreading as quickly.
The threat of the slugs already had a good foothold so spreading the disease would only slow the slug advance but would give humanity a fighting chance at survival.