**Core Concept**
The underlying principle being tested is the comparison of the **basic reproduction number (R0)** of Covid-19 and influenza, which determines their potential for spread. Covid-19, caused by the **SARS-CoV-2 virus**, has been a major global concern due to its high transmissibility and severity.

**Why the Correct Answer is Right**
Although influenza can spread faster, Covid-19 has a higher **case fatality rate** and can lead to severe **respiratory failure**, making it a significant concern. The **R0 value** of Covid-19 is also relatively high, allowing it to spread quickly in susceptible populations.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the question is about the global concern of Covid-19, not its treatment.
**Option B:** Incorrect as it does not address the comparison with influenza.
**Option C:** Incorrect because it does not consider the severity and transmissibility of Covid-19.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **R0 value** and **case fatality rate** are crucial in determining the global concern of an infectious disease.

**Correct Answer:** D. Covid-19 has a higher case fatality rate and can cause severe respiratory failure.