Python OOP Composition pattern: passing a class instance to another one attribute

Question

Goal

I learn composition OOP pattern with Python and I want to define a relation HAS-A from one class to another

Test

Here the Person class has a illness property with the help of the Medical class :

@dataclass
class Medical:
    DISEASES: ClassVar[list] = [
        'flue',
        'polio',
        'covid']
    _disease: str = None

    @property
    def disease(self):
        return self._disease

    @disease.setter
    def disease(self, value):
        if value in type(self).DISEASES:
            self._disease = value


class Person(object):
    def __init__(self, name: str, age: int, illness=Medical()):
        self.name = name
        self.age = age
        self._illness = illness

    @property
    def illness(self):
        return self._illness.disease

    def set_illness(self, value):
        self._illness.disease = value

Issue

When instantiating the first Person object, it's going fine. But the one after, the Medical class is not responding as intended and I can find why :

joe = Person("joe", 12)
vars(joe)
>>> {'name': 'joe', 'age': 12, '_illness': Medical(_disease=None)}

joe.set_illness('flue')
vars(joe)
>>> {'name': 'joe', 'age': 12, '_illness': Medical(_disease='flue')}

bill = Person("bill", 13)
vars(bill)
>>> {'name': 'bill', 'age': 13, '_illness': Medical(_disease='flue')}  # should be `'_illness': Medical(_disease=None)`