Not all the lice you see can lay eggs. Some may be male lice, others unfertilised females, these don't lay eggs. When combing hair parents sometimes spot large dult lice in the hair and fear they may be in the grip of a head lice outbreak but nothing else happens. They have probably found a male or unfertilised female. If you suddenly find an adult (large) louse on previously clean hair it has almost certainly just climbed onto this head from someone else. Large adults can't just appear. If a mixture of large and small lice is found then a full-scale infestation is present. When little lice are found they must have hatched on that head juvenile lice do not travel from head to head they are just too weak and too small.