Orchids are troubled by all kinds of pests and disease infestations. Apart from the creepy crawly ones, even winged visitors create a lot of nuisance, so much so that at times I had to keep guard (literally!) against crows. They would pull out the coconut fibres which were used to hold orchids to the small pieces log. Smaller birds like the Mynas would strip the orchids of the moss coverings. Though, I would like to add here that not all birds are harmful, some are friendly or even downright helpful (as in eating the insects attacking the orchids). Some act as pollinators and we have a number of orchids which attract their winged visitors by mimicry.
Molluscs like slugs and snails also cause a lot of harm to orchids, especially the potted ones, because of easy accessibility. Only orchids growing in hanging pots or baskets are safe from them. Slugs and snails become active at night, when they sneak in to eat away the softer parts of the plant. The easiest way to stop them from wreaking havoc is by circling the pots with a line of salt. You can also use bleaching powder in place of salt, but before doing that please check whether its use is not banned in your area. No slug or snail dare cross thus line, as salt can be harmful for slugs/snails.
Ants can also put an end to your efforts of growing orchids. The moist and warms conditions around the orchid roots are quite congenial for them. I found them building nests inside the coconut husks and underneath the small pebbles in the pots. Once I even noticed a whole colony of ants shifting with their eggs into my orchid pots! Once inside they eat the roots and also harbour harmful bacteria and virus. A very effective way to drive away ants is to spray Kerosene oil on to the plants. I did not notice any harmful effect of Kerosene on orchids and was quite relieved to see ants vanish. Kerosene also drives away other pests like crows, slugs etc.
Fungal, viral and bacterial infections are very common to orchids, mainly because of similar the growing conditions. Keep the plant parts such as leaves, flowers and stem dry, while watering, as wet parts attract fungal growth. If infected by virus or bacteria, immediately remove the infected part or segregate the whole plant. Personally, I am not in favour of using any kind of insecticide, pesticide or even a weedicide. So far I am not aware of any ill effects of Kerosene oil, but this also has been used quite sparingly. I have not used any other chemical repellent till date and I would rather stick to the principle of prevention being better than cure. So take care, be healthy and live wise.