Other aerobic organisms are Klebsiella sp, Haemophilus influenza, Streptococcus viridans, Eikenella corrodens, Enterobacteriaceae,[4] and salmonella sp.[7] Occurrences of AIT are most common in patients with prior thyroid disease such as Hashimoto's thyroiditis or thyroid cancer. The most common cause of infection in children is a congenital abnormality such as pyriform sinus fistula.[4] In most cases, the infection originates in the piriform sinus and spreads to the thyroid via the fistula.[8] In many reported cases of AIT the infection occurs following an upper respiratory tract infection. One study found that of the reported cases of AIT, 66% occurred after an acute illness involving the upper respiratory tract.[6] Although the rates of infection are still very low, cases of AIT have been on the rise in recent years due to the higher occurrence of immune-compromised patients.[citation needed] Other causes of AIT are commonly due to contamination from an outside source and are included below.