Examples include infectious diseases, illnesses caused by insects and exposure to biological and chemical substances or environmental pollution. Adverse hygiene conditions, long working hours, limited opportunities to relax and living long-term in a heightened state of alertness and tension are factors encountered during missions and can cause health damage. Exposure to health risks, or a combination thereof, can be extraordinarily draining, physically and mentally. Most service personnel do not experience any adverse consequences on this account after their deployment has ended. That said, some service personnel will develop symptoms after their deployment. These will sometimes be physical symptoms for which no clear cause can be found. Such symptoms can include headaches, memory problems, impaired concentration, dizziness, muscle and joint pain, sleep disorders, fatigue, skin disorders and digestive problems.