I’ve written before about how Our Microbial Overlords love playing jokes on us. Whether it’s transubstantiation myths or making cat owners wreck their cars, they do like a laugh at our expense. But now that Passover is on us once more I’d like to suggest an alternate narrative to the Four Questions many readers will be familiar with.

Let’s park Freud’s contention that Moses was a bit of a tosser (a textbook example of the kettle calling the pot black) or that Moshe’s taste for barbarism, slavery, wars of conquest and polygamy probably wouldn’t be considered too politically correct nowadays. We also need to ignore the scant historical evidence supporting biblical accounts of either the plagues or the Israelite escape from slavery in Egypt, or that there were ever large numbers of Israelites in Egypt at all.

So, let’s suspend any disbelief and accept the biblical description of 600,000 Israelite men (it doesn’t say how many women and children so far as I can see) leaving Egypt and mooching about in one of the most inhospitable regions on earth and – under Moshe’s leadership – surviving against all the odds. Which are considerable.

As ever, we shall concentrate on microbiological explanations for the Ten Plagues – and the contention that they may be one of the best gags Our Microbial Overlords have pulled on us. Ever.

Cast your mind back to a time where folks didn’t know where the sun went at night and – as now – more than 95% of Egypt’s population clung to the fertile banks of the Nile, whose floods periodically silted up the land and made it fertile – but with none of the predictability brought by the Aswan dams. So, they were used to a marginal existence and the occasional disaster.

But not on the scale of the ten calamities that, according to the Book of Exodus, were inflicted upon Egypt to persuade Pharaoh to release the ill-treated Israelites from slavery…

I will strike the water of the Nile, and it will be changed into blood. The fish in the Nile will die, and the river will stink and the Egyptians will not be able to drink its water.

Some claim a volcanic eruption in Ethiopia occurred which might have caused the river to either heat up thus killing the fish or possibly pumping it full of sulphurous goop. Or both. It has also been proposed that red dust from a cometary impact might have been responsible but I don’t buy either.

So-called ‘red tides’ caused by algal blooms in water courses are now well-charaterised and understood; the algae responsible contain red pigment and some species are also extremely toxic to fish. One good candidate for the biblical plague is a dinoflagellate called Pfiesteria has been responsible for many mass fish deaths. Pfiesteria secretes neurotoxins which narcotise fish so it can eat away their still-living flesh. As one might expect, this does not end well for the fish.

So, a Pfiesteria (or a similar freshwater dinoflagellate) bloom would turn the Nile red due to its characteristic pigment, an effect helped by the blood of the dying fish oozing from deep, open sores. And the water would have become so toxic as to be undrinkable. In recent years such ‘red tides’ have hit the USA, Hong Kong, the Mediterranean and elsewhere causing environmental catastrophes. All you need is a combination of excessive sewage discharge or agricultural runoff leading to nutrient-rich waters and some unseasonably warm weather.

And if thou refuse to let them go, behold, I will smite all thy borders with frogs:

And the river shall bring forth frogs abundantly, which shall go up and come into thine house, and into thy bedchamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens and into thy kneading troughs:

And the frogs shall come up both on thee, and upon thy people, and upon all thy servants.

So, due to the algal bloom no more fish. No fish to eat frogspawn would lead to unusually large numbers of the slimy (but tasty) little blighters, all of whom would have been keen to escape the now toxic waters of the Nile. But the bible talks about an awful lot of frogs – and frogs aren’t too well adapted to spend too much time away from water, especially in somewhere as arid as Egypt.

The word used in the bible is ‘tsefardea’, a catch-all description for frogs and toads of all persuasions so let’s look at the toad genus Bufo. Such toads are common throughout the world and produce huge numbers of eggs — hundreds of thousands from a single individual. So, populations of Bufo can rise from it being relatively rare to many millions in a heartbeat, all they need are the right conditions. Unlike most frogs and many toad species they also seek sources of light and heat to hunt the insects that they rely on for food. The biblical description above of the plague of frogs seems to match Bufo pretty well.

And the Lord said unto Moses, Say unto Aaron, Stretch out thy rod, and smite the dust of the land, that it may become lice throughout all the land of Egypt.

And they did so; for Aaron stretched out his hand with his rod, and smote the dust of the earth, and it became lice in man, and in beast; all the dust of the land became lice throughout all the land of Egypt.

(There’s lots of rod stroking in this part of the book of Exodus, fnarr fnarr. Paging Dr Freud!)

So, you have an unsustainable increase in the toad population. Which will therefore die off in pretty short order – as the bible describes: they “died out of the houses, out of the villages, and out of the fields. And they gathered them together upon heaps: and the land stank.”

Any sudden imbalance in any ecosystem will lead to unpredictable but interrelated increases and decreases in the populations of other species. The dinoflagellate bloom killed the fish; the death of the fish produced an increase in the frog / toad population followed by a population collapse, which in turn led to an upsurge in the insect population, which was free to breed unhindered in the absence of an important natural predator – frogs and toads.

Given the biblical description I think it’s unlikely these were actually fleas; More likely midges. Midge larvae feed on those of Our Microbial Overlords found in decaying stuff (like dead toads) about half-a-dozen species of midge (Culicoides) and about the same of sand fly (Phlebotomus) have been catalogued in Egypt.

Culicoides canithorax is a biting midge associated with both human and animal viral diseases – which are of relevance later – so I’ve got a fiver on that one. For our current purposes I think the lice can be lumped in with the fourth plague – flies – for they arise in similar situations.

Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.

And the Lord did so; and there came a grievous swarm of flies into the house of Pharaoh, and into his servants’ houses, and into all the land of Egypt: the land was corrupted by reason of the swarm of flies.

So, given the general shit, corruption and putrefaction going down, it’s hardly news there are going to be shitloads of flies arsing about. But for a proper plague you need not just any old fly like yer Musca domestica, you need crtitters like tsetse or stable flies, either of which leave open puncture wounds when they bite and thus the risk of secondary infection looms large, linking nicely with subsequent plagues. I can’t recall seeing tsetse as far north as Egypt and they’re pretty unmistakeable – about the size of a Nissan Micra and about as difficult to swat.

Behold, the hand of the Lord is upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen, and upon the sheep: there shall be a very grievous murrain.

And the Lord did that thing on the morrow, and all the cattle of Egypt died: but of the cattle of the children of Israel died not one.

So, now it gets interesting. An epizootic outbreak (a disease that affects animals but not humans) narrows things down a lot. Plus it only affects certain ungulates: there is no mention of domestic pets, birds, amphibians, reptiles, goats etc. Which rules out Anthrax, Babesiosis, Surra (an animal version of sleeping sickness), Rift Valley fever, Rinderpest or Foot and Mouth disease.

This has all the hallmarks of an insect-borne virus such as African Horse Sickness (which does not affect ruminants) or Bluetongue – which does. These are spread by the Culicoides midge (see ‘Lice’ above) which don’t roam far hence the Israelites’ flocks (in the land of Goshen) would have been spared – precisely as it says in the bible.

And it shall become small dust in all the land of Egypt, and shall be a boil breaking forth with blains upon man, and upon beast, throughout all the land of Egypt.

And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man and upon beast.

So, now we have boils and blains affecting both animals and humans. Anthrax, strep, staph could all do this but the most likely candidate is Burkholderia mallei, the bug responsible for glanders, a highly contagious infection which can be airborne, spread by direct contact or via vectors such as flies.

First described by Aristotle in 330BCE and used as a biological warfare agent in WW1, it is still found today throughout the Middle East and Africa. It infects both animals and humans. It spreads via the lymph causing the lymph nodes to swell and suppurate — hence the name, glanders — and fits the description of a plague of boils and blains extremely well. Again, its most likely carrier would have been the stable fly.

So there was hail, and fire mingled with the hail, very grievous, such as there was none like it in all the land of Egypt since it became a nation.

And the hail smote throughout all the land of Egypt all that was in the field, both man and beast; and the hail smote every herb of the field, and brake every tree of the field. Only in the land of Goshen, where the children of Israel were, was there no hail.

Is there a meteorologist in the house? Not my field. Sorry. So there was this big storm. Given these people had no idea where the sun went at night, a big storm must have been caused by the bloke in the sky. Obvious, innit.

And the Lord said unto Moses, Stretch out thine hand over the land of Egypt for the locusts, that they may come upon the land of Egypt, and eat every herb of the land, even all that the hail hath left. And Moses stretched forth his rod over the land of Egypt, and the Lord brought an east wind upon the land all that day, and all that night; and when it was morning, the east wind brought the locusts.

And the locusts went up over all the land of Egypt, and rested in all the coasts of Egypt; very grievous were they; before them there were no such locusts as they, neither after them shall be such. For they covered the face of the whole earth, so that the land was darkened; and they did eat every herb of the land, and all the fruit of the trees which the hail had left; and there remained not any green thing in the trees, or in the herbs of the field, throughout all the land of Egypt.

I’m no entomologist either but I saw a swarm of Schistocerca gregaria – the desert locust – once and in its gregarious phase  it’s a chilling thing to witness.

So, they’ve had a massive storm and now locusts. So what do you do? Gather up what you can and stash it away from the locusts. Remember that bit.

And the Lord said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt. And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:

They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings.

Sandstorms? Khamsin storm? Solar eclipse? Sky dark with locusts? Take your pick. I’m a microbiologist, remember?

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.


So, you can tell people there’s an invisible man in the sky who created the world in seven days and they’ll believe you. But tell them paint is wet and they just have to touch it. I’ve written about the false dichotomy of science and religion before and as a scientist I just have to touch the wet paint. That’s when I’m not licking windows. And all that lead paint that tasted so sweet when I was a kid did me no harm at all.

But while ‘proper’ science is about testing stuff sometimes it’s good fun just to speculate. And it could be that the plagues did happen in the way that I surmise above or it could be in the way described in a book written a millennium later by a bunch of blokes who had no idea where the sun went at night.

Everyone needs to believe something. I believe I’ll have a beer. Or rather four glasses of wine while leaning to the left, obviously.

Chag kasher v’same’ach, everyone…