The only way to determine if polio persists is to have trained observers on the lookout for it. This is 'surveillance', the intelligence network underpinning the entire eradication initiative. It pinpoints where wild poliovirus is circulating, as well as the type of virus and its origin. Once transmission is halted, surveillance will help verify complete polio eradication. Surveillance is a straightforward but intensive process. Health officials in every country monitor their populations for polio outbreaks. The tell-tale sign is a specific type of crippling called acute flaccid paralysis (AFP). Any child under 15 who has a 'floppy' limb is investigated, even though diseases other than polio might have caused it. If AFP is found, faecal samples, which must be kept cold, are taken from the child and rushed to an accredited laboratory for testing. Until this highly sensitive network was established, health officials had to estimate the world's total number of polio cases. When the Initiative started in 1988, they calculated that only 1 of 10 polio cases had been identified. Now, they can not only identify almost all polio cases, but can also identify the exact strain and origin of the virus. Even in the absence of wild poliovirus, surveillance has three main goals: detecting at least 1 case of AFP per 100,000 children under 15 (this accounts for the statistical probability of other types of paralysis); collecting adequate stool specimens from at least 80 per cent of AFP cases; and testing all specimens at a WHO-accredited laboratory. For this, a 147-laboratory global network is now in place. The US Centers for Disease Control and Prevention has been instrumental in establishing and monitoring this network. Mop-ups are focused vaccination campaigns that back up routine immunization and National Immunization Days. They specifically focus on places that are high-risk areas or where surveillance detects a rogue virus. For example, India is 'mopping up' in the remaining areas with relatively high rates of polio, in addition to carrying out NIDs. If surveillance finds polio circulating in a particular area, mop-ups targeting at least one million children are immediately organized. Vaccination teams go to every house, giving two doses of vaccine to all children under five, regardless of their immunization status. When deployed against rogue viruses, mop-up campaigns are comparable to fighting brushfires: quick deployments designed to contain unexpected outbreaks. In 1998, in the last recorded case of paralysis in Europe – two-year-old Melik Minas in Turkey -- mop-up teams immediately blanketed the south-east of the country, where Melik lived. They also covered adjacent regions of Iran and Iraq.		© Sebastião Salgado United States of America A scientist views a wild poliovirus strain called a 'virus tree' on a computer screen, at the Centers for Disease Control and Prevention in Atlanta. Strains from countries around the world are sent here to be stored and genetically identified. This includes pinpointing the exact geographical origin of each strain, a crucial factor in mapping follow-up immunization strategies.