We want to take advantage of the warm sunny weather and the longer days, but it's hard to enjoy them when we feel all tired and achy and stuffy. Most colds, and other more serious illnesses like influenza and measles, are caused by viruses, and there is no real cure. Viruses are small, simple particles that can't live and reproduce on their own. But inside a living cell, viruses become active organisms that can multiply very quickly. Viruses are often carried by air currents and are transported by body fluids into the cells.
Most viruses reproduce in certain kinds of cells.
For example, viruses that cause colds make themselves at home in cells of the human respiratory tract. A virus can't survive on its own - it must enter the cell of another living thing to live and reproduce. Some viruses don't seem to cause problems for us at all - others result in the flu or the common cold. You'd have to use an electron microscope to see a virus - they are shaped like little rods or spheres. The core of the virus has genetic material that enables it to duplicate itself.