Being genetically predisposed to bipolar disorder doesn´t guarantee that you´ll experience symptoms. Genetically predisposition only makes you susceptible to developing bipolar disorder. Think of it as a susceptible to skin cancer: If you are born with pale skin, you may be susceptible to extreme sunburn and skin cancer, but if you prudently avoid overexposure to the sun, you can prevent the cancer.
The same is true of bipolar disorder. You may have the susceptibility, but if you live a stress-free existence and take care of yourself physically, you may be able to live symptom free. You may be also one of the lucky few who have the susceptibility but don´t experience the symptoms.
Scientists don´t know all the factors that can activate bipolar disorder. Stress is well documented as a trigger, but other biological events, such as viruses may be involved. The severity of the underlying susceptibility is also important. Some forms are more sensitive to stress than others.
Source: Bipolar Disorder, candida Fink, MD, Joe Kraynak