This could explain why dark skin evolved in humans and animals living in tropical areas, where the threat of disease is greatest.
However, darker skin also contains large numbers of melanin cells in areas that are hardly ever exposed to sun's rays - such as the throat and nasal passages.
Research has also shown that melanin is actually a poor "sunscreen" and does not protect well against ultraviolet radiation.
New theory
New Scientist magazine reports that Dr James Mackintosh, an independent biologist based in Sydney, has come up with a new theory to be published in the Journal of Theoretical Biology.
White soldiers were three times as likely to contract "jungle sores", a skin disease caused by streptococcus pyogenes, compared with their black comrades.
Dr Mackintosh believes that melanin performs a similar function in humans and other large vertebrates.
In mammals, melanin is contained inside bladders called melanosomes. Larger, more numerous melanosomes make for darker skin.
Dr Mackintosh believes that melanosomes might engulf invading micro-organisms and use enzymes to kill them.
Sticky molecule
Melanosomes from human skin have been shown to inhibit micro-organisms in laboratory tests.
Dr Mackintosh said: "Melanin is a sticky molecule. The bacteria and fungi get all tangled up and it stops them from proliferating."
Also, a protein called attractin is known to regulate both melanisation and immunity in humans, suggesting a link between the two.
Dr Anders Møller, an evolutionary ecologist from the CNRS, France's centre for scientific research in Paris, said the theory was a "very good bet".
Prehistoric days
"It solves a lot of problems with these other theories."
Dr Mackintosh believes his theory also explains why we do not all have black skin. Melanin is made from the amino acid tyrosine which is also needed to build proteins.
He believes that in prehistoric days when food was scarce in cold, dry areas, tyrosine was probably conserved to make essential proteins.
It was only worthwhile converting it into extra melanin in the warm, damp tropics where food was abundant and diseases rampant.