Infrared tracking, also known as infrared homing, refers to a passive missile guidance system , which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers", since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines, and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background. 
Most drugs of abuse are addictive. Addiction is a chronic, relapsing disease characterized by compulsive drug seeking and use despite negative consequences and by long-lasting changes in the brain. People who are addicted have strong cravings for the drug, making it difficult to stop using. Most drugs alter a person’s thinking and judgment, which can increase the risk of injury or death from drugged driving or infectious diseases (., HIV/AIDS, hepatitis) from unsafe sexual practices or needle sharing. Drug use during pregnancy can lead to neonatal abstinence syndrome, a condition in which a baby can suffer from dependence and withdrawal symptoms after birth. Pregnancy-related issues are listed in the chart below for drugs where there is enough scientific evidence to connect the drug use to negative effects. However, most drugs could potentially harm an unborn baby.