Why is it difficult to treat viruses

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.
It is possible to be vaccinated against some of the major disease-causing viruses (such as measles and polio), as well as bacterial diseases such as Hemophilus influenza Type b (Hib), tetanus and whooping cough.