In his Tafsire Surah Baqarah (Chapter 2 of the Holy Quran) for the 49th verse: 'And remember, We delivered you from the people of Pharaoh…'

Ali Muhammad Shirazi, The Bab writes:

'Surely the progeny of Allah (i.e. progeny of Mohammed) in their status have an apparent (physical) body. Their bodies are stronger than the hearts of the people of Paradise. So much so that unless someone kills them they will not die. This is because their bodies are like that (secured) and they do not undergo any change (resulting in natural death). That is how Allah's Proof - Mohammed bin Hasan (Imam Mahdi) is alive today.'

Conclusion
Here are some deductions from the above statement of the Bab:

1. The members of Prophet Muhammad's (s.a.w.) progeny have physical bodies. This obviously includes Imam Mahdi. This is an important admission by Bab because subsequent Bahai leaders like Baha and Abdul Baha as also present day leaders claim that Imam Mahdi was only a spirit (and not a body) that reincarnated in the Bab. Apparently Bab, the centre of this reincarnation does not think so.
   
   

2. The members of the holy progeny don't possess ordinary bodies like lesser mortals. Their bodies are so resilient that even the hearts of the Paradise dwellers appear weak in comparison. Again obviously, Imam Mahdi is the possessor of such a strong body. Obviously his body is strong enough to withstand a few lashes which is more than what you can say for Bab's whining at a few harmless strokes.
   
   

3. Their bodies being of an extraordinary nature explains why it's not easy for them to die by natural causes. Something extraordinary needs to happen before they depart from this world. That explains why according to Shia traditions, all the Imams from the holy progeny were either felled by the sword or poisoned. If it weren't for these fatal blows, they would have lived forever unless Allah decided to make them taste death as He has promised every soul.
   
   

4. This is exactly what Bab has concluded about the existence of Imam Mahdi. He admits that Imam Mahdi being made from the same body as the other Imams cannot succumb to natural causes and is alive till date because none has had the opportunity to kill him either by sword or by poison. In fact, according to the Shias one reason why Allah has ordained Ghaybat for Imam Mahdi is to protect his life from his enemies. So Ghaybat in a way has secured Imam Mahdi's life and has ensured that he continues to live for as long as Allah wills.