Others have pointed out that when he appeared on North Korean television in July last year, Kim's lips were twisted, suggesting facial paralysis symptoms.
His apparent weight loss has also been a subject of many guesses. Some South Korean reports cited pancreatic cancer, while others infer that he is going through a rigorous diet program to prevent another stroke.
"All this is because the world fears what might happen post-Kim Jong Il," said Park Tae Gyun, professor of international studies at Seoul National University. "If he passes away without setting up a stable power structure surrounding his son, the international community would have to deal with hard line military forces with nuclear capability."
Wookyung Chloe Jung and Yijeong Lee contributed to this report