In the first of its three stages which potentially fatal STI can be treated and cured with antibiotics?

The STI you are referring to is syphilis. In its primary stage, syphilis can be effectively treated and cured with antibiotics, typically penicillin. Early diagnosis and treatment are crucial to prevent progression to the more severe stages of the disease, which can lead to serious health complications if left untreated. Regular screening and safe practices are essential for prevention.