Reference: Rudolph's Pediatrics
St Johns Pediatric Dentistry
Teething is a time of stress in parents life. Historically teething got the blame for many infant illness. It is important to know that teething on its own is unlikely to make a baby sick. Studies showed that in 98% of babies who where sick during teething, the illness was due to something else, not the teeth! It is also important to remember that at the same age that teeth start to erupt baby's will start loosing mommy's antibodies (natural protection they received from mom while in the womb). This will make them more susceptible to infections. Also at this age they start exploring and putting objects in their mouths, this also increase the risk of infections.
Tooth eruption can be delayed due to problems like vitamin D deficiency and also things like malocclusion (position of teeth are wrong). In some families it is genetic.
See the picture below on when to expect teeth!