Suppose a group of astronauts travelled to another Earth-like planet. They get off their space-craft, take off their space suits and begin sunbathing, unclothed. As their immune system has not evolved on this planet, would they immediately die as a result of exposure to alien microbes? So, even if the planet has the same atmospheric composition, temperature and the same gravity as Earth, would alien microbes still make it inhospitable to human life? Also, suppose aliens came to Earth and did the same thing, would they also die because their immune systems have not evolved on Earth, so they would not have immunity to microbes on Earth?