Gets on membranes. Viruses are small particles of dna or RNA wrapped in protein. They are found abundantly in mucous from other people, in an effort to spread themselves around. When they are introduced to your mucous membranes, the genetic material incorporates itself into your cells, making your cells (usually lining the nose) into making more virus. Touching eyes and nose passages with unwashed hands spreads most.
Being exposed... Simply walk outside; and being around people...The are viruses everywhere and at all times; it is how our immune systems respond to and process these attackers that determine whether or not we get "sick" (cold, flu, etc.) good hand hygiene (washing with soap and water) is one of the best defenses.
Contact. Viruses are often spread by direct contact-either touching something/someone contaminated with it, and touching your mucus membranes (nose, eyes, mouth) or by inhaling the virus. Some remain contagious for a brief period after leaving the host, others can persist for hours on objects. Good handwashing and avoiding contact with infected individuals is your best defense.