Influenza viruses and others continually change over time, usually by mutation. This change enables the virus to evade the immune system of its host so that people are susceptible to influenza virus infection throughout their lives. Bacteria mutate in the same way and can also become resistant if overtreated with antibiotics.
Many human illnesses are caused by infection with either bacteria or viruses. Most bacterial diseases can be treated with antibiotics, although antibiotic-resistant strains are starting to emerge. Viruses pose a challenge to the body's immune system because they hide inside cells
Theory: <u>A theory is a carefully thought-out explanation for observations of the natural world that has been constructed using the scientific method, and which brings together many facts and hypotheses.</u>
