Three people have died on planes from Miami to London in separate incidents over the past two days.
A woman and a man died on British Airways (BA) flight 208 on Sunday - the company said the deaths were not thought to be linked.
And a 19-year-old woman, who is thought to have had an existing medical condition, died just before a Virgin Atlantic flight landed on Monday.
BA said it was "incredibly rare" to have two deaths on the same flight.
Its plane made an unscheduled stop in Halifax, Canada, on Sunday after a female passenger became ill.
Suspected meningitis reported
She was met by paramedics from the plane but died later in hospital.
After the plane took to the air, a man became ill and witnesses said part of the passengers' cabin was curtained off. He died shortly before landing at Heathrow Airport.
A BA spokesman said they could not confirm the passenger's illness, but The Guardian newspaper reported he had suspected meningitis.
On Monday a 19-year-old woman died shortly before landing at Heathrow, despite efforts to save her by a doctor and the cabin crew.
She boarded the flight with 291 other passengers on Sunday evening.
Police and paramedics met the flight and a post-mortem examination is being carried out.