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Misconceptions
"Zero gravity" or even nearly zero gravity is a misnomer, and does not really exist anywhere in the universe. Gravity is an omnipresent, if sometimes subtle force. What is usually meant is instead better described as weightlessness. For example, the force of gravity at 100 km altitude in Earth's orbit is actually 97 percent of what it is for people standing on the ground. The difference is that conditions in orbit negate the perception of gravity.
The Physics of Weightlessness
Our main perception of gravity is due to forces reacting against it. For example, you perceive gravity most because of the pressure exerted by the ground, furniture and other solids that push back against it and support your mass. Acceleration is another such effect. Weightlessness occurs when all such forces, including gravity, become evenly and uniformly applied on an object. Gravity usually does act uniformly, so eliminating all the other constraints results in weightlessness and usually occurs during free fall. Free fall is motion where their is little or no aerodynamic drag, and all acceleration is caused by gravity. These conditions exist naturally in space.
Weightless Simulations
There are ways to simulate weightlessness that do not involve going into space. Since the early 1970s, aircraft flying on six-mile long parabolic arcs have been used to create 25-30 seconds of weightlessness. The NASA aircraft that have filled this role are traditionally dubbed "vomit comets" because the induce Space Adaptation Syndrome.
Space Adaptation Syndrome
Better known as "space sickness," Space Adaptation Syndrome is the first negative effect of weightlessness that an astronaut might encounter. It is encountered both on "vomit comets" and on space missions. It is usually limited to nausea and vomiting, but sometimes induces vertigo, headaches, and lethargy. Space sickness afflicts a little under half of all people who go into space, but even the worst cases last no more than three days.
Long-term Health Issues
The two biggest problems with weightlessness and the human body are what the loss of resistance does to muscles and bones. Muscles begin to atrophy, and bones experience osteopenia or the loss of bone mass. These things happen because the lack of resistance tells your body to stop maintaining its bone and muscle tissue. Bone loss is predictable at a little more than 1 percent loss per month in space. It also produces fluid redistribution and slowing of the cardiovascular system: as gravity no longer provides resistance, the heart does not have to work as hard, and fluids are no longer pulled down and into the extremities, causing them to collect in the torso. Other more serious issues include balance disorders, decreased red blood cell production and immune system difficulties. Most of the health problems associated with long duration weightlessness will clear up upon return to normal gravity conditions, excepting truly serious bone loss. The current standard for long duration missions in space are the duty tours aboard the International Space Station, which are set for six months and pose no risk to astronauts.