Contamination doesn't even have to be self-replicating to be a major concern. Many organometallic compounds are exceedingly lethal and accumulate in biotic life. If alien tissue uses any to a significant degree, it would have disastrous consequences.

Obviously, the dangers of runaway self-replication makes even the smallest samples of primitive life potentially more deadly. And, while primitive life is far more likely in the universe at large, it is astronomically unlikely to reach our solar system undirected.