R. Jacob said in R. Johanan's name: We may cure ourselves with all things, save with the wood of the asherah (a tree worship for idolatry).
That is, when a human life is in danger, any means of curing is permitted – even if, for example, it involved a product from a non-kosher animal like a pig.
When Rabin came, (to Babylon from Israel) he said in R. Johanan's name: We may cure [i.e., save] ourselves with all [forbidden] things, except idolatry, incest (sexual immorality), and murder.
Idolatry, as we have stated (with the wood of the asharah). Incest and murder, as it was taught: Rabbi said: For as when a man riseth against his neighbour, and slayeth him, even so is this matter (Deut. 22:26).
The Deuteronomy verse quoted has to do with rape and the punishment of the aggressor. The full section is:
But if a man finds a betrothed girl in the field, and the man forces her, and lies with her; then the man only who lay with her shall die; But to the girl you shall do nothing; there is in the girl no sin deserving death; for as when a man rises against his neighbor, and slays him, so is this matter; For he found her in the field, and the betrothed girl cried, and there was no one to save her. (ibid. 25-27)
Now, this is a section not without some controversy, because (a) if this happens in the city (where if she cried someone would have heard her) they are both killed and (b) if she is not betrothed, the penalty is merely financial. Still, the point is that in this verse (22:26) rape is directly connected with murder.
Now, what connection has a murderer with a betrothed maiden? Thus this comes to throw light, and is itself illumined. The murderer is compared to a betrothed maiden: just as a betrothed maiden must be saved [from dishonor] at the cost of his [her ravisher's] life, so [in the case of] a murderer, he [the victim] must be saved at the cost of his [the attacker's] life. Conversely, a betrothed maiden [is learned] from a murderer: just as [in the case of] murder, one must be slain rather than transgress, so a betrothed maiden must be slain yet not transgress.
Ok, that last part is also problematic. The point here, though, is that sexual immorality is on the same level as murder.
And how do we know it of murder itself? (i.e. that one must accept death rather than commit murder)
It is common sense. Even as one who came before Raba and said to him: The governor of my town has ordered me, ‘Go and kill So-and-so, if not, I will kill you.’ He answered him: ‘Let him kill you rather than that you should commit murder; what [reason] do you see [for thinking] that your blood is redder? Perhaps his blood is redder.’