Plants and plant products are imported from foreign countries by means of freight, personal luggage and parcel post etc. They are subject to plant quarantine to prevent a possible invasion of pests which may inhabit these plants. A crop or herd of livestock under quarantine means that an agronomist, pathologist, or veterinarian has found positive evidence of some kind of (usually) pathogenic infection or infestation in the crop that requires absolute isolation from any other crop or herd. Absolute isolation means that the appropriate measure must, by law, be taken to prevent any further spread of the disease, pest, or condition.
A few examples of quarantineable diseases are: BSE ("mad cow") in cattle, scrapie in sheep, erysipelas in pigs, white rot in onions, rhizomania in sugar beet, and potato cyst nematode in potato.
Imported plants are commonly inspected for quarantine pests. If plants could possibly bear serious pests, they are inspected more closely. If any quarantine pests are found at the inspection, measures are taken on the consignment such as sterilization, sorting, elimination of the pests or destruction (incineration) of the plants. Recent progress in marine containerization and air freight services has made it possible to import large quantities of fresh fruits and vegetables from abroad. Due to their nature, seeds are easy to move over unlimited distance. They are also easy to store in viable conditions for short or lenthy periods of time. They can harbour organisms of quarantine significance such as fungi, bacteria, viruses, nematodes and insects and can be contaminated by unwanted weed seeds.