The fourth programme in a six-part series on the human body. Looks at some of the dangerous microbes that live on or inside the human body, and our increasing vulnerability to disease. Examines the dramatic increase in allergic diseases such as asthma, eczema and hayfever, and considers the so-called `hygiene hypothesis' put forward to explain this, which suggests that the human body's immune system overreacts to these allergic conditions due to insufficient exposure to everyday dirt. Shows that exposure to bacteria can save lives. One of the case studies featured is the successful treatment of a brain tumour using the Herpes Simplex virus.