At the end of 2019 a new disease emerged from the city of Wuhan in China. The World Health Organisation called this new disease COVID-19. The first case of COVID-19 in Croatia was confirmed on 25th February 2020. On 16th March 2020, all educational institutions were closed down in the whole of Croatia. Online schooling was instituted for all students. There are many definitions of adolescence. To simplify, adolescence is a period from the start of puberty until adult age. The World Health Organisation defines mental health as the bedrock of personal wellbeing and effective functioning. Adolescents were subjected to prolonged isolation from their peers, cessation of sports and other social activities, to the challenges of remote learning, personal illness or illness in the family, or even death. The course of the pandemic yielded high rates of depression, anxiety and post-traumatic syndromes. Higher rates of neglect and abuse, as well as disturbances in family relations, isolation and loneliness were reported. Long COVID is a collection of symptoms that last for weeks or months in patients after they are infected with the SARS-CoV-2 virus, and there are pronounced effects on mental health. There are initiatives and programmes created by adolescents for adolescents as support groups. Research shows that the majority of adolescents adhere to recommended public health COVID policies and recommendations. The pandemic has an effect on individuals and the society as a whole and causes anxiety, stress, stigma and xenophobia. Young people may experience anxiety, distress, social isolation and they can have short-term and long-term impacts on their mental health. Epidemics lead to stigmatization. Except negative impacts of the COVID-19 pandemic and lockdown, some positive effects have been observed. Seven studies noted positive impacts on the health of adolescents. Limited fast food intake, increased intake of nuts, fruit and vegetables has been observed. Increased physical activity, lower rates of a sedentary lifestyle and increased effectiveness of online schooling result in positive developement of mental and physical health.