mutate- to change from one thing or type of thing into another. 

contagious – can be spread between people or animals. 

infectious – able to infect someone. 

disease – illness of people, animals, plants, caused by infection or failure of health. 

flu –infectious illness that causes fever and headache. 

pneumonia – a serious illness in which one or both lungs (легкие) become red and filled with liquid (жидкость). 

incubation period – the time between the infection  and the appearance of the first sings. 

immune system – the body’s organs that fight illness and disease. 

respiratory tract – the parts of the body (mouth, nose, throat (горло) and lungs) that air passes through during person’s breathing. 

carrier – a person who can give a disease to other people but may not show any signs of having the disease.