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An apple a day might not keep the doctor away... but if is correct, a little bit of biodiversity might be the trick.

The link between biodiversity and health is a topic we've been into previously on this blog - almost two years ago, in fact.

That concerned a paper showing that rates of (also known as bilharzia, and something you definitely don't want to catch) could be reduced by environmental management that preserved a diversity of snails in ponds and rivers.

The schistosomes that cause the disease spend part of their lives in snails - and preserving the diversity appeared to mean that parasites would enter snail species in which they couldn't develop, leading to fewer of the things emerging to infect people.

The white-footed mouse is an efficient carrier of the Lyme Disease bacterium

That study is among those featured in the Nature piece, which reviews evidence from many different human diseases - malaria, hantavirus, West Nile virus - as well as some affecting frogs and plants.

So across the board, does biodiversity provide some kind of protection against disease?

In general, yes, the scientists conclude - and for a number of reasons.

, for example, is carried by mosquitos. Infected mosquitos bite birds, the virus multiplies within the bird, resulting in a higher virus load, more infected mosqitos and eventually, more infected people.

Several studies in the US have shown that when the diversity of bird species is high, the risk of humans being infected is relatively low - perhaps, again, because mosquitos are busy pumping their viral load into birds where the virus will not multiply.

You might argue that the impact of diversity loss ought to depend on which species are becoming depleted.

If it's a disease carrier that's disappearing, the net impact on human health ought to be positive.

What this analysis suggests is that in many different cases, the survivors are species that do carry disease.

Again in the north-eastern US, for example, - a very nasty tick-borne infection - is carried and indeed amplified by the white-footed mouse, an abundant species and one that appears to be adaptable enough to survive the fragmentation of forests.

The opossum, on the other hand, which is a poor host for the bacterium, doesn't handle degraded land too well and moves on - potentially leading to an increase in disease transmission.

Thinking back through recent history, there are pretty obvious examples where this general rule (if rule it is) breaks down.

Clearing the Kenyan highlands of mosquitos during the colonial era obviously brought down insect diversity - but had a hugely beneficial impact on malaria.

Another limitation of the analysis is that it only looks at infectious diseases. Monoculture crop plantations might drive away animals that would otherwise decoy pathogens - but if they provide greater volumes of food, the net impact on human health might still be positive.

One of the intriguing possibilities signalled in the review paper - which, in the interests of clarity, I should signal is written by 13 scientists from a variety of institutions in the US and from the Zoological Society of London - is whether you could use information about ecology and how it's changing to identify potential disease hotspots.

Could you spot places where biological diversity is being truncated, and model what's likely to happen in terms of vectors and hosts and contact with people?

Alternatively - could such modelling studies provide new reasons to conserve some of nature's remaining diversity?

Bearing in mind what - namely, that

"By 2020, at the latest, biodiversity values have been integrated into national and local development and poverty reduction strategies and planning processes and are being incorporatedinto national accounting, as appropriate, and reporting systems"

- you could assume it will provide additional incentives for conservation.

A 1993 paper estimated that Lyme Disease cost the US about $1bn per year... and beside sums like that, the cost of keeping a few tracts of opossum-friendly forest intact might look like money well spent.

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