COVID-19 is primarily a respiratory droplet infection. This means that the virus can be transmitted from one individual to another via the air when an infected person coughs. This, however, requires reasonably close proximity and/or prolonged contact.
The secondary route of infection is via the hands – usually when a person touches a contaminated surface (viral particles can persist on surfaces for several days). However, as coronavirus enters the body via the mouth, nose or eyes, a person must touch their face for this route of infection to be successful.
While washing one’s hands thoroughly for at least 20 seconds will prevent transmission in this way, this is not always a possibility, such as when travelling or commuting. It is therefore essential to avoid touching one’s own face, as this eliminates the only known means of spread that doesn’t require close proximity to an infected person.
Here is how Chris Whittey, Chief Medical Officer for England, explained the subject to MPs last Thursday.
This is the key intermediate step. Just touching [a contaminated surface] will not give you the virus. It is if you touch it and then touch your face, having not washed your hands between them. So if you go into the tube and touch a railing, that’s fine – just be aware of what you do with your hands, don’t touch your face, and wash your hands.Professor Chris Whitty, Chief Medical Officer for England, UK Parliment Health and Social Care Committee
UK Parliment Health and Social Care Committee: UK's plans to tackle possible coronavirus pandemic examined: https://committees.parliament.uk/committee/81/health-and-social-care-committee/news/145352/uks-plans-to-tackle-possible-coronavirus-pandemic-examined/
World Health Organisation Q&A on coronaviruses (COVID-19): https://www.who.int/news-room/q-a-detail/q-a-coronaviruses