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.