One of the dangerous things about space travel is that you will be exposed to space radiation. We don't see the effects of this radiation on Earth because our planet is surrounded by the atmosphere and magnetic field. Most of these gases that make up the atmosphere protect us by absorbing these radiations.
But when you travel to space, you will be exposed to these radiations because space lacks the type of atmosphere that surrounds the Earth. For instance, when you travel to Mars, you will be exposed to ten times the radiation that comes to the Earth.
One of the effects of radiation exposure is that it may increase your chances of having cancer. Space travel can also damage your CNS (central nervous system), and this may result in acute effects. It can later cause behavior changes and changes in cognitive functions. It can also result in anorexia, nausea, fatigue, vomiting, etc.