This is an area of study that is still under investigation. Extended periods of weightlessness do have effects on the human body. There is usually a period of initial nausea and vertigo that has been termed "space sickness" while the body adjusts to the change in gravity. After the body adapts, these symptoms disappear. But extended periods of weightlessness do have negative symptoms. Deterioration and weakening of the muscle and bone tissues starts to occur. Regular exercise can help to fight off these effects. A weakened immune system, a decreased red blood cell count, eyesight issues, and a redistribution of body fluids are other long term effects. Most of these effects in most people go away once they return to Earth.