Why is salmonella most common in summer?

Answer 1

There could be more than one reason, but summer is warmer and more conducive to bacterial growth. It is also a time when people are more active outdoors and tend to eat at picnics. Temperature-abused food will grow more Salmonella. The exposure to the out-of-doors could cause more contamination of food. People being active outside don't always wash their hands and that could be a source of contamination.

A study in Georgia found that the ground water contains more Salmonella during the summer - no surprise there.

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Answer 2

Within the normal temperature range, bacterial growth increase with temperature. So, in the summer, foods that are left un-refrigerated can get contaminated by bacteria which will grow to levels where they can cause food poisoning.