 A healthy plant can synthesize proteins in order to grow, absorb water and nutrients and translocate them to where they are needed, perform photosynthesis and lose byproducts through the roots and through transpiration, and reproduce in order to bear fruit or seeds. But just like humans and animals, plants can get sick and result in visible symptoms. Just like humans, plants can be attacked by insects and pathogens such as viruses, bacteria and fungi. Ill health can also be caused by deficiencies in nutrient ions which are vital for growth and development. Now we will look at some specific examples. Plants can suffer massive infestations. One insect example, the aphid, swamps a plant in vast numbers. Each aphid has piercing mouth parts that tap into the leaf so that they can feed on the sap. This results in a wilted or distorted leaf that cannot photosynthesize correctly. A byproduct of all this eating is a honeydew that they exude from their hind gut which often is collected by ant soldiers who guard over their flock. If this isn't bad enough for the plant, secondary infection can occur with aphids transferring viruses through their mouth parts and leaving holes in leaves for other pathogens to make use of. Another problem comes in the form of honeydew residues, which is the perfect source of food for fungal colonies such as black sooty mould. A classic virus is this tobacco mosaic virus or TMV. It has a rod-like structure consisting of a coat of proteins positioned helically around a single strand of RNA and was actually the first virus to be described by Adolf Meier. But can enter wound sites. Once it's in, TMV can hijack plant cells to aid its own replication. As the name suggests, the virus leaves a mosaic pattern on the leaves and also causes malting, cellular death, stunting and leaf curling. This virus can devastate tomato crops. An example of fungal infection in plants is black spot fungal disease. It can infect any plant with fleshy leaves and it's really common in roses. This is a big problem for ornamental growers where looks at everything, a spotty plant can affect its value. Sometimes, disease is not caused by an infection but by a nutrient deficiency. In fact, plants can be damaged by a range of iron deficiency conditions. For example, stunted growth caused by nitrogen deficiency. Nitrate ions are needed for protein synthesis and are therefore important for growth and repair as well as all of the plant's enzyme-controlled functions. Another example of nutrient deficiency is a change of leaf colour called chlorosis which is caused by magnesium deficiency. Magnesium ions are needed to make chlorophyll which is essential for photosynthesis and energy production. Many plant disease symptoms are nonspecific like spots on the leaves so it's not always easy to identify what's causing the symptoms. Identification can be made in a range of ways. For example, reference to a gardening manual or website. Taking infected plants or a sample of a plant to a laboratory or using testing kits that contain monoclonal antibodies. Monoclonal antibodies work by binding to parts of the infected viruses, bacteria or fungus which is suspected of causing the infection. Sometimes plants do not show symptoms because they, like humans, have evolved a range of ways to avoid and fight infection. The first barrier to infection is the plant's preformed defences. These can be physical barriers like cellulose cell walls, tough waxy cuticles on leaves or layers of dead cells around stems which fall off like bark on trees. There may be mechanical adaptations such as thorns and hairs or leaves which droop or curl when touched, mimicry to trick animals. All the responses to infection can have a chemical basis such as antibacterial chemicals even weaker news. Symptoms like the toxins in nettles that cause us to come up in blisters and plant defence molecules that are produced as a response to attack. These chemicals can affect the pathogen or insect directly or induce cell death to stop the attacker in its tracks. Plants are so clever they can even send signals in the air to call on insect predators that will take out an attacker. In summary, plants have a range of physical and chemical barriers to prevent infection but they can become infected with bacterial, viral, fungal pathogens or insect pests. If this happens, the plant can produce chemicals in an attempt to limit the infection. Plants can also get sick if their soil is deficient in nutrients.