The people who are at higher risk of severe illness include older adults, with risk increasing by age, and people are immunosuppressed or have serious chronic medical conditions such as heart disease, diabetes, cancer, and lung disease.

References:

1. https://www.cdc.gov/coronavirus/2019-ncov/specific-groups/high-risk-complications.html 

2. https://www.who.int/news-room/q-a-detail/q-a-coronaviruses