Also, as written, the object "target", is passed "by value" to the function, "creature::attack". That means the object inside the function is a new object with the same "value" as the object passed. In other words, it's a copy. So you are correct - changes made to that copy don't affect the object which was originally passed to the function.

One way to make the function work with the actual object passed, rather than making a copy to work with, is to make the function call pass the object "by reference", like this:

void creature::attack(creature & target) {...}

And the function declaration needs the same change.

class creature
{
  public:
    virtual void attack(creature & target);
    ...
}

Did you set the initial health after creating creature?
Sounds like uninitialized variable to me.

This works fine for me:

#include <iostream>
using namespace std;

class creature
{
public:
virtual void attack(creature target);
int health;
int damage;
string name;
void sethealth(int x) {health = x;};
};
void creature::attack(creature target)
{
target.sethealth(target.health-damage);
cout << target.name << " now has " << target.health << " health. " << endl;
}

int main (){

creature player;
creature enemy;

player.name = "Nick";
enemy.name = "Bokoblin";


player.sethealth(100);
enemy.sethealth(100);

cin >> player.damage;

player.attack(enemy);


}