2 The number of bacteria in 1 ml of drug \(A\) has a Poisson distribution with mean 0.5. The number of the same bacteria in 1 ml of drug \(B\) has a Poisson distribution with mean 0.75 . A mixture of these drugs used to treat a particular disease consists of 1.4 ml of drug \(A\) and 1.2 ml of drug \(B\). Bacteria in the drugs will cause infection in a patient if 5 or more bacteria are injected.

1. Calculate the probability that, in a sample of 20 patients treated with the mixture, infection will occur in no more than one patient.
2. State an assumption required for the validity of the calculation.