Thomas Cook agent Susan Cooper and her husband, who died in Egypt five years ago, were staying at a hotel being fumigated to kill bedbugs, an inquest has heard.
The couple, aged 63 and 69 from Burnley in Lancashire, were on holiday in Hurghada with their daughter and three grandchildren.
The inquest in Blackburn heard the Coopers fell ill at the Aqua Magic Hotel during the early hours and died the next day in August 2018.
The inquest was told that a report pointed to possible exposure to an “infectious biological agent or toxic chemicals”.
The report suggested neither carbon monoxide poisoning nor food poisoning caused the couple’s deaths, Blackburn Coroner’s Court heard.
A tourist from Germany said he reported a bedbug infestation in the room next door to the Coopers and it was then treated with pesticide, referred to as Lambda, the BBC reported.
The two rooms had an adjoining door but this was kept locked.
Their daughter, Kelly Ormerod, described her parents as fit and healthy and had been enjoying a “brilliant” holiday.
Mr Cooper, a builder, and his wife, Susan, a cashier at a bureau de change in a Thomas Cook agency in Burnley, took several holidays a year.
Mrs Cooper had been to the same hotel in April that year, described it as “fabulous” and decided to go back with her family.
The hearing was adjourned until today (Wednesday).