There's some truth to what he posted, but how long the fly has been in contact probably should be taken into consideration as well.
Should you throw away food if a fly lands on it? Expert reveals all
Writing on a blog on the University's website, Dr Webb said: 'In most instances, spotting a fly on your food doesn't mean you need to throw it out,' he wrote.
'While there is little doubt that flies can carry bacteria, viruses and parasites from waste to our food, a single touchdown is unlikely to trigger a chain reaction leading to illness for the average healthy person'
Flies are notorious for spreading disease because of their messy habits, Dr Webb said.
'Flies that land out of sight and then wander about for a few minutes vomiting and pooping on your food or food preparation area are more of a concern.
'The more time passes, the greater the chance of pathogens left behind by the flies growing and multiplying on our food. That's when health risks increase.'
Dr Webb said flies in the city are typically more hygienic than those in the country who frequently come in contact with dead animals and animal waste.
'For the most part, insecticides and improved hygiene standards assist in fly control (in the city) and minimise the risk of contact with contaminated substances,' he said.
Dr Jeff Scott added that whether he abandoned his food would depend on where the fly had been.
Dr Scott said: 'That will depend on where the fly has been before it landed on your sandwich.
'If it had been feeding on manure prior to sharing my sandwich, my appetite would be greatly diminished.'
Adult house flies (musca domestica) carry more than 100 human and animal diseases.
These include salmonella, anthrax, typhoid, tuberculosis, cholera and diarrhoea as well as a host of parasites including pinworms, roundworms, hookworms and tapeworms.
Food hygiene experts advise that while a solitary fly may not pose much of a health risk, the presence of flies can indicate poor hygiene.
Dr Jeff Scott, a Cornell University professor of entomology, has explained why flies pose a danger to human health.
'Anything that comes out of an animal, such as bacteria and viruses, house flies can take from that waste and deposit on your sandwich,' he said.
'House flies are the movers of any disgusting pathogenic microorganism you can think of.'
Recent research has found that flies are fussier than we might think and avoid certain rotten fruits .
The insects have dedicated brain circuits that can pick up toxic moulds that might be harmful to the insects, Germany's Max Planck Institute found.
A chemical, geosmin, that is produced by toxic moulds and bacteria, is repulsive to flies.