Stage 3 is a "waiting" game. Your DNA accumulation is slow but rockets at the end. Pop red and orange bubbles to collect DNA, and devolve any symptoms that mutate to get a bonus.
An open-world style map filled with spore clouds and sniper-type infected. parasite infection game walkthrough
The main quest sends you to St. Lazarus Cathedral to meet “The Hive Mind”—a colony of infected who achieved symbiosis. Stage 3 is a "waiting" game
While "Parasite Infection" is the main game, you might find other games with "parasite" in their name. Here are two notable ones: parasite infection game walkthrough