Pimple like bumps on chicken head and skin

[JESAYA]

Hi I'm raising cornish cross broiler about 5 weeks old. My chickens have Pimple like bumps that appears on the head and through out the body. Some chickens appear sick while others are behave healthy eventhoughtheyhave the same symptoms.
Some have bold spot on the head, found one with a swollen head and 2 can't walk even though they are not overweight. The wings of 2 appear bloody as if injured. In the last week I have lost about 20 chickens. The chickens are not coughing nor do they appear to have any breathing problems.

I'm bedding on sand that is replaced during mid cycle for my six weeks term and at the end.
The coop is made of corogated iron in the sides a concrete floor and a chicken mesh wire in the middle covered by an insect net.

The chicks come vaccinated against Newcastle, IB4,Gumboro and ILT. We vaccine them again at 14 days against Newcastle,Gumboro and Bronchitis.

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[PippinTheChicken]

If some aren't infected, separate them in case it's spreading