Every monsoon, I meet patients who walk into my clinic with high fever and a worried look. One conversation, in particular, repeats itself year after year. “Doctor, I had dengue a few years ago. It can’t be dengue again, can it?” A few days later, when the test comes back positive, the surprise is often followed by another question: “I thought I was immune.”It is an understandable assumption but unfortunately, it is one of the biggest misconceptions about dengue. Unlike infections such as chickenpox, recovering from dengue does not provide lifelong protection. A person, who has recovered from one episode, can develop dengue again if infected with a different dengue virus serotype. More importantly, a repeat infection may carry a higher risk of severe disease in some individuals. This does not mean that everyone who gets dengue a second time will become critically ill, but it does mean that symptoms should never be taken lightly and early medical evaluation becomes even more important.Why can dengue occur more than once?Dengue is caused by four closely related but distinct viruses, known as serotypes — DENV-1, DENV-2, DENV-3 and DENV-4. Infection with one serotype provides long-term immunity only against that particular strain. It does not protect against the other three. If a person is later infected with a different serotype, they can develop dengue again. Since multiple serotypes circulate simultaneously in India, repeat infections are not uncommon.Also Read | India’s first dengue vaccine approved: DCGI clears Takeda’s Qdenga for those aged 4 to 60Why can a second infection sometimes be more severe?The increased risk during a second infection is linked to the body’s immune response. After the first dengue infection, the immune system produces antibodies against that specific serotype. If a different serotype causes a subsequent infection, these antibodies can recognise the virus but may not neutralise it effectively. Instead, they may inadvertently help the virus enter certain immune cells, allowing it to multiply more efficiently. This phenomenon is known as antibody-dependent enhancement (ADE).The result can be a more intense inflammatory response, increasing the permeability of blood vessels and causing plasma to leak into surrounding tissues. In severe cases, this may lead to low blood pressure, fluid accumulation in the chest or abdomen, bleeding manifestations, dengue shock syndrome or organ involvement.Fortunately, not every second infection becomes severe. Many patients recover uneventfully with supportive care. However, anyone with a previous history of dengue should be especially vigilant if they develop symptoms again.Must Read | People with type 2 diabetes face higher risk of dengue, warns review; experts weigh inThe critical phase begins when the fever settlesOne of the common mistakes people make is assuming that once the fever starts coming down, the illness is over. In dengue, the opposite may be true.Story continues below this adThe period between the third and seventh day of illness — when the fever begins to subside — is often the most critical. This is when plasma leakage, bleeding complications and shock are most likely to occur in patients who develop severe dengue. Close monitoring during this phase is essential, particularly for those with a previous history of dengue.Know the warning signsThe early symptoms of dengue include high fever, headache, pain behind the eyes, body aches, nausea and rash. However, certain warning signs require immediate medical attention. These include persistent vomiting, severe abdominal pain, bleeding from the gums or nose, blood in vomit or stools, increasing drowsiness, difficulty breathing, cold or clammy skin, restlessness and reduced urine output. These symptoms should never be ignored or managed at home without medical supervision.Platelet count is only one part of the pictureMany people equate dengue severity with a falling platelet count. While platelet monitoring is important, it is only one piece of the puzzle.Doctors also assess hydration status, blood pressure, haematocrit, evidence of plasma leakage and the patient’s overall clinical condition before deciding on treatment. Some patients with very low platelet counts remain stable, while others may develop serious complications despite platelet counts that are not critically low.Story continues below this adDon't Miss | Dengue fever and heart health: Expert warns of hidden cardiac risks to noteTreatment decisions should therefore always be guided by the complete clinical picture—not by a single laboratory value.Prevention remains the best defenceHaving had dengue once is not a reason to lower your guard. Since infection with another serotype is still possible, preventing mosquito bites remains the most effective way to reduce risk.Using mosquito repellents, wearing clothing that covers the arms and legs, installing screens where possible, and preventing water stagnation in and around homes are simple but effective measures to reduce mosquito breeding and transmission.The takeawayA previous episode of dengue does not guarantee lifelong protection. In fact, a second infection with a different serotype may carry a greater risk of complications because of the way the immune system responds. The aim of understanding this is not to create fear, but to encourage vigilance.Story continues below this adEarly diagnosis, adequate hydration, recognition of warning signs and timely medical care — especially during the critical phase when the fever begins to settle — can make all the difference. With prompt supportive treatment, the vast majority of patients, including those experiencing dengue for a second time, recover well.(Dr Chatterjee is senior consultant, Internal Medicine, Indraprastha Apollo Hospital, Delhi)