Fifty-four staff at a meat processing plant at Bridge of Allan in Stirlingshire have now been confirmed as contracting the rare infection, Q fever.
NHS Forth Valley set up an Outbreak Control Team to tackle the outbreak at the plant operated by family-run firm Scotbeef, which supplies several leading retailers throughout the UK.
Scotbeef has been working with the Helath and Safety Executive to establish the source of the outbreak.
Q fever is diagnosed by blood test. The test only shows a positive result two to four weeks after the onset of the illness when antibodies are present in the blood. By this time most people have made a full recovery.
In some cases people will have been unaware of the infection. There are no new reports of people getting ill, according to a statement from NHS Forth Valley.
In humans, the symptoms of acute Q fever are similar to flu – they begin to show about 2-3 weeks after infection.