These one-cell organisms are responsible for illnesses such as strep throat<span>, urinary tract infections and </span>tuberculosis<span>. Viruses. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to </span>AIDS<span>. Fungi. </span>
Food is pushed through the digestive tract by muscle contractions called
<span>peristalsis.</span>
A social psychologist would be interested in how groups interact.
