Man has crossed oceans and both benefited and suffered the consequences, but now stands on the brink of space. Humans, who reproduce much more slowly and less often than the other organisms they will intentionally and unintentionally take with them, and whose immune systems react slowly to assaults by unfamiliar enemies, will be hard put to survive the biological chaos that will often follow. Although space travelers will all be inoculated against all the standard diseases, and vaccines will be taken along into the void, effective vaccines do not exist for every disease. Large organisms, like humans, pigs, and cabbages, are gardens of microorganisms, some of which are indeed very likely essential to their health. In addition, it is to be noted that a large organism absolutely free of bacterial occupants would be an open invitation to new occupants.