I belive the answer to the first one is no I would have to assume a habitual zone is one area on a planet. for example, an air conditioned building vs. the Savvanah dessert. However I am not sure maybe other answers will help more on this one. Sorry.... Earth-like planet would of course need water, plant life, animals, oxygen, fossil fuels etc. A challenge we would face on a travel to another liveable planet: do we have engough transportation? engough fuel to make it? How long can we live on this planet? Who or what else is present here? etc. Yes I do think they should discover a planet with earth like qualities befor investing in tools to get us to them. So money is not wasted. (make sure to put this into your own words )
The main big one would be disease and infections. Trench fever, trench foot and trench mouth were common diseases people got when involved in trench warfare. Trench foot happens when the feet are exposed for long periods of time to cold water and mud. Trench fever is caused when body ice transfers from one place to another. Overcrowding conditions make this prone to happen. It causes headaches, sore muscles, skin lesions on the chest and backs as well as fever.
I hope this helps :)
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