Subject: Why didn't early explorers take new diseases home with them?

In my history classes we often read about how european explorers to North 
America, South America and Australia introduced diseases to the indigenous 
populations that had no resistance to these diseases, resulting in widespread 
deaths.  Some books attribute extensive depopulation  to these diseases. My 
question is why were no diseases transmitted the opposite way, from the "new" 
worlds to europe with similar results of widespread deaths?

Re: Why didn't early explorers take new diseases home with them?