Nausea, malaise, swelling of hands and feet (sometimes the brain), shortness of breath, occasionally fatal. Not symptoms you would necessarily want to be characteristic of your football team.
This is what can happen when you rise above 2400 metres (8000 feet). Next week, San Lorenzo will take on the Bolivian team Real PotosÃÂ Ã¢ÂÂ¦at a height of 4,800 metres (15,500 feet). Needless to say San Lorenzo aren't relishing the task ahead of them.
They have hired oxygen tanks for their players, put together a strategy meaning they will take several days to acclimatise, practising in the lower Santa Cruz, before riding along the dirt roads up to PotosÃÂ on the day of the game.
They have also had to take yellow fever vaccinations, leading to the temporary illness, ironically, of most of their medical staff.
Santiago Hirsig, a San Lorenzo midfielder, has already played in PotosÃÂ, but with his previous team Arsenal (who got hammered last night 6-0 by Brazilians Fluminense). "It made me feel terrible," he said. "Lots of the players fainted after the game. And the journey there is also hard work, so you already arrive tired."
FIFA have already tried to ban playing football above 2500 metres (8200 feet), ruling out any games in Bolivia at all, but also Quito and BogotÃÂ¡. Fortunately for the Bolivian sides FIFA backed down from the proposal after a tongue lashing from Bolivian president, and football fan, Evo Morales.
But given the general quality of the grossly underfunded teams in one of the world's poorest countries, it is the only advantage they have.