Most potted orchids, which include Cattleyas, Dendrobiums, Paphiopedilums get frequently infected with pests and diseases during summers. There are lot of factors during this time of the year which contribute to the spread of disease. Orchids are damaged by pests like aphids, beetles, caterpillars, mealy bugs, mites, snails and slugs, scale insects, thrips; apart from these bacterial and viruses also attack orchid plants.

Damage caused to the orchids
• Some suck plant sap, thereby decreasing the growth vigour
• Some pests are carriers of bacteria and virus
• Cause damage to flowers
• Young plant are more susceptible

Prevention is the best policy
Summer is the best time for pests and diseases. Some things to keep in mind to prevent pest or disease outbreak:
• Check regularly for pests and diseases
• Maintain good air circulation
• There should be no water stagnation
• Lighting should be adequate, with no dark spaces
• In case of infection, immediately isolate the diseased plant

Control
• For aphids: Oil or soap sprays can be used in case the infection is not very extensive. Chemical sprays are also helpful in controlling aphids, e.g. Pyrethins. Sticky yellow coloured insect strips are useful in detecting aphid infection on orchids.
• For bacteria and viruses: Always clean the tools with alcohol and use sterilized potting mix (if possible). Most viruses are spread by aphids, so keep checking for aphid infection.
• Other insects like caterpillars: Dipel contains Bt (a friendly bacteria) which kills the caterpillars. Carbaryl wettable powder can also be used.
• For spider mites: Low and very high temperatures are deterrent to growth of mites. Spider mites can also be controlled by oil sprays.
• For scale insects and mealy bugs: Scale insects can be controlled by spraying with alcohol or introducing eco-friendly biological control methods like introducing the parasitic wasps which eat away scale insects. Systemic sprays should be used as the last resort to control scale insects.
• For snails and slugs: I have found circling orchid pots with salt can be an effective and eco-friendly way to put off the crawling insects like snails and slugs.
• For chewing insects like beetles and thrips: Beetles can be removed manually and thrips can be caught on the sticky insect strips.

Precaution
• Avoid spraying any kind of chemicals on flowering orchids. Some chemical sprays carry statement of being safe to use even during flowering stage, but it would be better to avoid.

Source
Alford, David, V. Pests of Ornamental Trees, Shrubs and Flowers: A Colour Handbook, Second Edition.