Can the repentance of someone who has committed the abomination of Zina four times be accepted, despite repeatedly breaking a covenant with Allah not to return to it, and is it possible for the disease to be transmitted to him?

If you repent to Allah with a sincere repentance that fulfills its conditions of cessation, remorse, and determination not to return [to the sin], then Allah will accept your repentance and forgive your sin, as stated in His Almighty saying: "Say, 'O My servants who have transgressed against themselves [by committing sins], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Strive to fulfill repentance, distance yourself from everything that leads to falling into sin, conceal your sin and do not tell anyone, and seek a righteous environment that helps you repent. As for the covenant you made with Allah not to commit fornication, it is considered an oath. Since you broke it, in addition to repentance, you must offer an expiation for breaking an oath. Regarding your question about the transmission of disease, it should be directed to specialists.