There are more than 2,500 different strains of Salmonella and symptoms include diarrhea, stomach cramps, nausea, vomiting or fever. The current outbreak is understood to have been caused by a strain known as Salmonella Newport.
Salmonella is most commonly caught from contaminated food, usually through meat or diary products and those infected are warned to take care to maintain their fluid levels.
Most get better without treatment but some require antibiotics or a stay in hospital if they become dehydrated. The very young and very old are most at risk of serious complications which can include septicaemia (blood poisoning) or a localised infection such as septic arthritis.
In the past the Food Standards Agency has advised consumers to wash all fruit and vegetables before eating as a preventative measure.
Symptoms usually occur within eight to 72 hours after eating a contaminated product.
Previous figures for the number of cases of Salmonella Newport recorded in the UK are 180 during last year, 221 during 2010 and 182 during 2009. The five year average is 197.
There are two possible routes of infection that have lead to the current outbreak. First, the surface of the watermelons could have been contaminated with Salmonella bacteria and this may have transferred onto the flesh of the melon during the cutting process.
Second, if the melons were stored or washed in contaminated water then Salmonella bacteria could have got into the flesh of the melon through the cut stem of the fruit.
Last year an outbreak of Salmonella Newport occured in Germany and the Netherlands. The cause was bean sprouts.
It is not uncommon that the source of outbreaks is not identified because foods contain multiple ingredients and identifying which is the contaminated component can be very hard.