What makes some COVID-19 variants more contagious than others?

The virus that causes COVID-19 is evolving, with new and more infectious variants taking hold. Last week, Victoria entered a seven-day lockdown due to another COVID-19 outbreak. That has now been extended for at least another seven days in Melbourne. But what makes this outbreak different from others is the spread of a 'highly infectious' variant that was first detected in India in October last year.