Abstract base class question C++

I I have an abstract base class in c++ as follows:

	class IMyBaseClass
	{
	public :
		virtual ~IMyBaseClass() {}
		virtual void AddSomeInformation( int param1, int param2) = 0;
		virtual void AddSomeMoreInformation( int param1) = 0;
	};

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When I go to implement a class with this interface as follows:

       class MyBaseClass : public IMyBaseClass
	{
	public :
		virtual ~IMyBaseClass() {}
		void AddSomeInformation( int param1, int param2);
		void AddSomeMoreInformation( int param1);
	};

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Do I have to include the word virtual before the function name in this class definition??
e.g.

virtual void AddSomeInformation( int param1, int param2);

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instead of just:

void AddSomeInformation( int param1, int param2);

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Can you explain your answer please?