Time to event and drop-out model

Dear NM-users. I am modeling the preventive effect of a drug by using a time-to-event approach (time to get a new parasite infection after treatment). Patients were treated once a month for 3 months, with a scheduled follow-up 1 month after the last treatment (and again if they were symptomatic during 2 additional months of follow-up) The PK has been modeled and fixed for the TTE-model. A constant hazard with sigmoid Emax drug effect was used to explain the time-to-new infection with a good RSEs and reasonable parameter estimates. However, in this study, the dropout events are not randomly distributed. After the third month (after last treatment), 30% of all patients dropped out and did not come back for the 1st follow-up visit (the average dropout rate is about 2% each month). Many of the patients (50%) that came back after the 1st follow-up visit had acquired a new infection. I therefore believe that many of the patients that were lost did not have an infection. I am wondering if I need/how to model the drop-outs or in any way compensate for the fact that only patients without infections dropped out? I would also like to ask if anyone know how to diagnose TTE-models (except VPCs)? Your comments and help is most appreciated. Thank you and Best Regards Palang Chotsiri PhD-student in Pharmacometrics Mahidol-Oxford Tropical Medicine Research Unit, Bangkok 10400, THAILAND