Plants "catch" a virus similar to the way we fall victim to a headcold or influenza. Sucking insects, such as aphids, can transfer virus diseases from plant to plant and should be controlled in the garden. All garden-grown plants, whether lily, iris or tulip have one or more viruses present in the tissues. Trouble doesn't start until several different viruses find a home in the same plant. A plant can grow quite happily with one or two different strains of virus, but will begin to show streaking and twisting in the leaves, and eventual death, if several more strains are added. This is similar to an office worker functioning with a simple headcold, but add the flu...
There is no cure for plant viruses, only prevention. Regularly inspect your garden throughout the growing season, bearing in mind that some cultivars may have suspicious-looking foliage early in the season, but look great later. This is due to envirnment, not virus. If in a group of recently planted lilies, all look great except one, keep an eye on the one which looks unhealthy, but check for outside factors, such as mechanical damage to the sprout, insect activity, or competition from overly aggressive trees, shrubs or ground covers. It is best to control aphids, and wait until early the next season before passing judgment. Relocating bulbs to better drained soil, adding humas, or providing Trace Elements will many times correct foliage concerns.
Note: The old-fashioned Tiger Lily (Lilium tigrinum); tall, orange recurved flowers is heavily infected with viruses, do not plant with other hybrids, keep isolated by at least 100 feet to be safe.