(18)Unavoidable and extraordinary circumstances include objective situations, for example warfare, other serious security problems, such as terrorism, significant risks to human health, such as the outbreak of a serious disease at the travel destination, or natural disasters, such as floods, earthquakes or weather conditions, which make it impossible to travel safely to the destination as agreed in the package travel contract. Unavoidable and extraordinary circumstances should not only cover circumstances which make it objectively impossible to perform a package but also circumstances which, without preventing such performance, mean that the package cannot be performed without exposing the travellers concerned to risks to their health and safety. In this context, certain risks may affect certain groups of travellers particularly severely due to their vulnerability.