Hi yeop, it has been quite some time since I follow up this thread. The phenomenon you experience is normal. People usually set their Level to 1 in the Ghost Ship stage just to collect Infection mission for others, a.k.a infection farming. You can always host the room without lowering your level to let others know that you want to play the full session.

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Added on May 7, 2012, 11:16 pm

It's quite hard to seek for any co-op session among players in Lowyat, you might better off request it on GameFAQ's board here.