An Otago University expert says botulism is extremely rare in New Zealand and Australia, with the last case in this country 28 years ago.
Fonterra says a botulism-causing bacterium may have contaminated three batches of whey protein used in infant formula.
Senior microbiology lecturer Heather Brooks says the bacteria causing botulism is common in soil and water, and people probably swallow it without realising.
She says it becomes dangerous only if the bacteria have been multiplying in food and produce a toxin that's swallowed.
She says in babies the toxin can multiply in their intestine, producing symptoms including sleepiness, poor feeding combined with constipation, an expressionless face and weak or high-pitched cry and drooling.
If a child is well and feeding normally, there should be no concern.
Dr Brooks says honey is the only infant food she's aware of that's been implicated in botulism.