About 3,000 people took part in the X-runner mud run, which involved open water swimming, at the National Water Sports Centre in Nottingham on 9 July.
The health body is trying to identify the cause of the illness which affected a number of competitors.
Event organisers said water quality test results had been "normal".
One runner named Sarah said she had been suffering from sickness, diarrhoea, headaches, fever and aching limbs and was still not well more than a week after the event.
She said test results had ruled out food poisoning and still believed the water quality was responsible for her symptoms.
Jane Renwick, from Derbyshire, said five of the seven in her team were struck down and she was unable to keep any food down for five days.
Many people also commented on Facebook about feeling unwell and some were not happy with X-runner's response.
As part of its investigation, Public Health England (PHE) has asked those affected to fill in a survey to help identify the cause of the illness and where it originated.
A PHE spokeswoman said in a statement that it was working with X-Runner and the trust that runs the National Water Sports Centre in an advisory capacity.
She said it was "unclear as to the cause of the illness and investigations are ongoing".
Last week X-runner said about 25 people contacted them after experiencing "a mild stomach upset" which it said can occur when swimming in open water.
It said it had also advised competitors of the risk beforehand during the sign-up for the event.
PHE has asked those affected to fill in its questionnaire by Sunday.
What did entrants have to do?
Public Health England is investigating why dozens of people suffered bouts of vomiting and diarrhoea after taking part in an "extreme" obstacle race.