Most of us think that when we step under a shower we're getting clean.We are, but we're also giving a home to lots of tiny little creatures we didn't even know about.          A showerhead carries thousands of bacteria   called Mycobacterium.These can cause problems like coughs and tiredness, and a general feeling of poorliness  .When you turn on the water, the bacteria go from the showerhead onto and into your body.          This is a finding of Norman R.Pace and his team at the University of Colorado, in the US.The scientists investigated bacteria in all kinds of human environment, including showers.          Pace's team looked at 45 showerheads in nine American cities.They discovered that 30 percent of them had large amounts of flying Mycobacterium.          But Pace said that they pose few threats to the health.Only those with a frail   immune   system might need to worry.          He told the New York Times that the bactertia are not as unpleasant as might be thought.He said that having a shower is no more dangerous than anything else we do in the morning.          But for those who feel sick about the idea of all those microorganisms  , he had some advice. Let the water run for 30 seconds before getting into the shower.Why? The number of bacteria is smaller than when the water is just turned on.If that seems like a waste of water, he added that you could also change your showerhead every few months. But Pace had good news too.He has also been testing the air in US subways.Apart from iron particles  , which are ground off the track by the wheels of trains, subway air is fresh.The reason is that a train's movement pumps fresh outdoor air into the tunnels . Pace explained that he wanted to understand the natural microbial  environments of public places.This kind of knowledge might help  _ the microbes to be used in a bioterrorist  attack. Pace's research into showers shows that   _  .
there is no reason to fear microorganism shower water contains much less bacteria after being left to run for 30 second having a shower in the morning is more dangerous than at other time of 45 showerheads surveyed in nine cities, 30 carried large amounts of Mycobacterium