And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts.
So, water supplies are undrinkable. Fish (vital protein source) wiped out, as are animal protein sources due to sundry pestilence, your beasts of burden are gone so fields are left untilled and what crops are left are either buried by a sandstorm and as for those you’ve saved…
Remember the locusts? You’ve got damp and damaged crops covered in locust shit with no time to dry or sort them before storing. So – again – take your pick. Typhoid fever (Salmonella typhi) or salmonellosis (Salmonella typhimurium) – probably the latter because it affects animals as well? But the description is of a pretty immediate sort of death, not a long drawn out one. And for that you need toxins (aaaaaaargh! the toxins!).
Mycotoxins are produced by fungi while growing on stuff like corn and other crops. The fungi themselves are not harmful, but the mycotoxins they produce can be lethal. I’m pretty sceptical about any health impact toxic fungal growth in modern buildings, and we need one that will selectively target the first born of both humans and livestock. Hmmmm…….
A possible reason for preferential impact on the firstborn is that the most dominant animals and humans may have had preferential access to the stored food supplies, and so they would have been most affected by the fatal toxins. Organisms such as Stachybotrys grow on plant matter, principally cellulose and the tricothecenes it produces have been linked to animal and human deaths in many countries. You only need tiny quantities of these toxins to cause sudden death. So using improperly stored, mouldy grain in the face of starvation might well have precipitated widespread illness and death.
The differential impact on the firstborn may have its explanation in the bible where it is mentioned the firstborn should not just be fed first but receive double rations of whatever is available. Also the first animals to feed would be the most dominant ones.
The Israelites could have escaped the worst effects this because those elements of the plague that caused the worst of the famine in the rest of Egypt hadn’t hit them so hard, and the key elements of the Passover Seder – new-born lamb, herbs and unleavened bread — would all have been safe from the potential for mycotoxin intoxication. It’s interesting that the bible has detailed injunctions on how to deal with the presence of certain kinds of mould in dwellings, including one apparently from Moshe’s own teachings which instructs Jews to destroy any dwelling in which mildew is found.