Shakespeare took the basic plot of Macbeth from a history of the British Isles.  He freely adapted this account, adding, changing and dramatising details at will.  The original historical story ran as follows:

1034- King Malcolm II murdered at Glamis by his fellow warlords.

His son Duncan subsequently killed his rivals and seized the throne.  Duncan then had two sons, called Malcolm and Donald.

Meanwhile Macbeth joined forces with Duncans half-brother, Thorfinn, who had a claim to the throne.  Together, Macbeth and Thorfinn defeated Duncan and split the kingdom between them.

There are many differences between the story told in Holinsheads Chronicles of England, Scotland and Ireland (Shakespeares main source of material for Macbeth).  The characters have been changed slightly, such as Lady Macbeth becoming more manipulative in Macbeth than she originally came across in history.  Shakespeare wanted to explore the human character, and to do this he looked at real acts it had committed, through history.  He wanted to look at evil, corruption and issues to do with power.  In fact, it may be the humanity of this play that means that it still appeals to people four hundred years after it was written.

And why the filthy? I hear you ask.  In the times in which Shakespeare was writing, they believed that bathing was bad for you.  Instead, people covered up the smell with waxes and perfumes.  This is why weddings are traditionally in the summer, as it was nearest to the Elizabethan and Jacobean people's annual bath!