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$R_0=1$ is the threshold separating outbreak decline from growth. If $R_0 < 1$, each case infects fewer than one other person on average, causing transmission chains to break and the outbreak to fade.

Conditions: The video states the growth side only until immunity builds up.; The example value shown is $R_0=2.5$.

$R_0=1$ is the threshold separating outbreak decline from growth. If $R_0 < 1$, each case infects fewer than one other person on average, causing transmission chains to break and the outbreak to fade.

Conditions: The video states the growth side only until immunity builds up.; The example value shown is $R_0=2.5$.

When $R_0 > 1$, each generation of infections is larger than the previous one because every case produces more than one new infection on average. This repeated multiplication by a factor greater than one results in exponential growth of the outbreak.

Conditions: Assumes the same average transmission per case across generations.; The worked example uses $R_0=2.5$.

When $R_0 > 1$, each generation of infections is larger than the previous one because every case produces more than one new infection on average. This repeated multiplication by a factor greater than one results in exponential growth of the outbreak.

Conditions: Assumes the same average transmission per case across generations.; The worked example uses $R_0=2.5$.