The reasoning: epidemiology
| Feature | Dengue | Chikungunya |
|---|---|---|
| Agent | Flavivirus, DENV 1–4 | Alphavirus (togavirus) |
| Vector | *Ae. aegypti (primary, urban), Ae. albopictus* | Same; E1‑A226V mutation adapted it to albopictus |
| Incubation (intrinsic / extrinsic) | 4–10 days / 8–12 days | 2–7 days |
| Immunity | Lifelong to the infecting serotype; cross‑protection short; secondary heterotypic infection ↑ severe dengue risk (antibody‑dependent enhancement) | Long‑lasting |
| Clinical | Febrile → critical (plasma leakage, around defervescence, days 3–7) → recovery | Fever, severe polyarthralgia (may persist for months), rash |
| Burden | Lakhs of reported cases annually in India; under‑reporting substantial | Large outbreaks (2006, 2016) |