Q4. How many serotypes of the dengue virus are there, and to which virus family does it belong?
Four; the family Flaviviridae (beyond the NCERT text; Wikipedia, facts in our words).
Q5. In an outbreak, 60 people fall sick with dengue. Using MedlinePlus’s proportion, about how many may develop severe dengue?
About 1 in 20 of those who fall sick: 60 ÷ 20 = 3 (beyond the NCERT text; MedlinePlus).
Q6. What can severe dengue cause, and when do its warning signs usually begin?
Shock, internal bleeding and even death. The warning signs, such as severe belly pain, repeated vomiting and bleeding from the nose or gums, typically appear 24–48 hours after the fever ends (beyond the NCERT text; MedlinePlus).
Q7. Why does NCERT recommend putting fish such as Gambusia in ponds?
They feed on mosquito larvae (NCERT page 134), so fewer larvae grow into the adult mosquitoes that spread disease (reasoned from NCERT page 134). Beyond the NCERT text, the mosquitofish (Gambusia affinis) has harmed small native fish where people brought it into waters outside its home range (Wikipedia, “Mosquitofish”, facts in our words).