[Valmont]

Uuhhm wait a minute...
How about you overloading operator< OUTSIDE the Patient class? Do you see why?

#ifndef PATIENT_H
#define PATIENT_H

class Patient
{	
public:
	Patient(){};
	~Patient(){};
	friend bool operator<(const Patient &T, const Patient &P);
public:
	void setPriority(int priority)
	{ itsPriority = priority; };
	int getPriority()
	{ return itsPriority; };
	void setTime(int time);
	int getTime();
	void setMisdiagnose(int misdiagnose);
	int getMisdiagnose();
private:
	int itsPatientNumber;
	int itsPriority;
	int itsTime;
	int itsMisdiagnose;	
};

bool operator <(const Patient &T, const Patient &P)
{
	if (T.itsPriority < P.itsPriority)
		return true;
	else if (T.itsPriority > P.itsPriority)
		return false;
	else if (T.itsTime < P.itsTime)
		return false;
	else 
		return true;
}

#endif //PATIENT_H

Do you also understand what the friend keyword means?
Also learn to place private(s) last in the class declaration. Details last