Even the humble cold can give off an odour, thanks to the thick bacteria-ridden mucus that ends up in the back of the throat.
The signs are not apparent to everyone - but some super-smellers are very sensitive to the odours.
Joy Milne, for example, noticed her husband`s smell had changed shortly before he was diagnosed with Parkinson`s disease.
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Humans can detect nearly 10,000 different smells.
Formed by chemicals in the air, they are absorbed by little hairs, made of extremely sensitive nerve fibres, hanging from the nose`s olfactory receptors.
And the human sense of smell is 10,000 times more sensitive than the sense of taste.
But dogs, as the old joke might have had it, smell even better.
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Media captionClaire Guest from Medical Detection Dogs says her dog Daisy helped detect her cancer: ``She looked concerned``
Their ability to detect four times as many odours as humans makes them a potential early warning system for a range of diseases.
Research suggesting dogs` could sniff out cancers, for example, was first published about 10 years ago.
And there have been many tales of dogs repeatedly sniffing an area of their owner`s body, only for it to turn out to be hiding a tumour.
What they are smelling are the "volatile molecules" given off by cells when they become cancerous.
Some studies suggest dogs can be 93% accurate.
Others suggest they can detect very small tumours before clinical tests can.
And yet more studies have produced mixed results.
Does cancer smell?
At Milton Keynes University Hospital, a small team has recently begun to collect human urine samples to test dogs` ability to detect the smell of prostate cancer.
The patients had symptoms such as difficulty urinating or a change in flow, which could turn out to be prostate, bladder or liver cancer.
Rowena Fletcher, head of research and development at the hospital, says the role of the dogs - which have been trained by Medical Detection Dogs - is to pick out samples that smell of cancer.
Further down the line, a clinical test will show if the dogs` diagnosis is correct.
She says the potential for using dogs in this way is far-reaching - even if it is not practical to have a dog in every surgery.
"We hope one day that there could be an electronic machine on every GP`s desk which could test a urine sample for diseases by smelling it," she says.
"But first we need to pick up the pattern of what the dogs are smelling."
And that`s the key. Dogs can`t tell us what their noses are detecting, but scientists believe that different cancers could produce different smells, although some might also be very similar.
Lab tests to understand what these highly-trained dogs are smelling could then inform the development of `electronic noses` to detect the same molecules. These might then give rise to better diagnostic tests in the future.
The potential for using smell to test for a wide range of diseases is huge, Ms Fletcher says.
Bacteria, cancers and chronic diseases could all have their own odour - which may be imperceptible to only the most sensitive humans, but obvious to dogs.
It may be possible in the future to use disease odours as the basis for a national screening programme or to test everybody at risk of a certain cancer in a particular age group.
However, there are fewer than 20 dogs in the UK trained to detect cancer at present. Training more will take more funding and time.
On the positive side, all dogs are eligible to be trained provided they are keen on searching and hunting.
Whatever their breed or size, it`s our four-legged friend`s astounding sense of smell which could unlock a whole new way of detecting human diseases.
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