Dr Raj Kiran, a resident doctor at the All India Institute of Medical Sciences (AIIMS), died on September 29 of dengue haemorrhagic fever. It was his second brush with dengue.
Vinod, 23, who died of dengue on Sunday had also been infected with the disease earlier.
Doctors say the majority of the 22 patients who have died of dengue in Delhi this year had had it before too.
"A person infected with one type of virus maintains lifelong immunity to that type but remains susceptible to infection from other strains of the dengue virus," said Dr Shiv Lal of the National Institute of Communicable Disease (NICD). "The second attack may also be more severe."
"As previous antibodies mingle with the current dengue infection, the virus multiplies faster inside the body," said Dr Anoop Misra, senior consultant, Fortis Hospital. "It releases toxins which lower the patient's blood pressure."
All four strains of the dengue virus — D1, D2, D3 and D4 — are active this year.
Thus people who have had dengue in the past should be extra careful. They should look out for warning signs like low blood pressure, recurrent vomiting, pain in the abdomen and sudden changes in body temperature.
Dr Misra said it was difficult to predict how each patient would react on contracting dengue for a second time. "By the third or fourth day, there may be external or internal bleeding — clear signs of haemorrhage," he said.
Dr Bir Singh, of AIIMS, said there was as yet no vaccine or treatment to prevent dengue haemorrhagic fever or the dengue shock syndrome.
"In the case of patients who have had a previous attack, the immune system reacts in such a way that blood vessels start to leak," he said. "There is no medicine to stop the bleeding. When vital organs have less blood supply than they need, multiple-organ failure results."
Doctors say patients unable to consume 3 litres of fluids a day should get hospitalised for intravenous rehydration.