Orchid viruses are one of the most complicated topics that the average hobbyist will consider. Hopefully you will never encounter the problem, but if you trade plants and divisions with friends it is entirely possible that you will eventually see viruses. There are many viruses which attack orchids, but the most prevalent are Odontoglossum ringspot virus (ORSV) and Cymbidium mosaic virus (CyMV). For some excellent photographs of the effects of these viruses, visit Agdia.
Symptoms of these two viruses vary somewhat by genus. Many plants will be asymptomatic (show no symptoms) but just never grow or flower well. Cattleya and Phalaenopsis will display a variegated look, or striping, in the color of the petals and sepals at time of flowering. Cattleya and Laelia will sometimes show spotting either yellow or brownish or reddish on their leaves. Yellow spotting and striping of leaves of any variety of orchid are symptoms which are suspicious and should be heeded. If you have an orchid with any of these symptoms, quarantine the plant away from others since viruses are very easy to transmit from one plant to another.
Viruses are the main reason that so much has been written about disinfecting tools and hands between plants. It is especially easy to transmit when the tool has been used for cutting a leaf or inflorescence of an infected plant and then used to cut another plant. Use a disposable razor blade for these chores to reduce the chances of passing on viruses. To disinfect tools use a 1:9 (one part bleach to nine parts water) bleach/water solution and dip your tools for at least one minute. Be sure to rinse tools to reduce the corrosive effects of the bleach on metal.
Virus testing can be done by sending a leaf to various testing laboratories or by ordering testing strips from a lab to do the test yourself. Agdia testing strips are available in groups of five for the hobbyist. In Europe visit Pocket Diagnostic for a similar product. Many orchid societies are ordering these strips for their members so that each person can get only as many as they need. Testing is done by taking a small sample of the orchid leaf or root, reducing the sample to a semi-liquid form and dipping a test strip in the sample. Results are read on the test strip. They are quite reliable if used correctly, however they are relatively expensive.