**Core Concept**
The H1N1 pandemic is caused by the influenza A virus subtype H1N1, a type of RNA virus that affects the respiratory system. The pandemic is typically declared when a new strain of the virus emerges, spreads rapidly across the globe, and causes significant morbidity and mortality.

**Why the Correct Answer is Right**
The World Health Organization (WHO) declares a pandemic when the following criteria are met: the virus has spread to at least two countries, there are sustained community outbreaks, and there is a significant increase in cases and deaths. This is often determined by the WHO's Pandemic Alert System, which uses a six-level alert system to track the spread of the virus. The system includes phases such as phase 1 (pandemic alert and response) and phase 6 (pandemic declaration).

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a pandemic is not declared based solely on the presence of a new strain of the virus. While the emergence of a new strain is a key factor, it is not the sole determining factor.
**Option B:** This option is incorrect because a pandemic is not declared based solely on the spread of the virus to multiple countries. While international spread is a key factor, it is not the sole determining factor.
**Option C:** This option is incorrect because a pandemic is not declared based solely on the increase in cases and deaths. While an increase in cases and deaths is a key factor, it is not the sole determining factor.

**Clinical Pearl / High-Yield Fact**
When a pandemic is declared, it is essential to implement public health measures such as vaccination, social distancing, and mask-wearing to slow the spread of the virus.

**Correct Answer: D.**