There are two types of service-connection

1. Direct and
2. Presumptive

There are three requirements to establish Direct service-connection for residuals of injuries and diseases;

1. In-service documentation of an injury or disease.
   

A current condition with a medical diagnosis.

"Before November 2000, when the VCAA was enacted, veterans had to obtain a medical diagnosis of a current disability on their own. The VA was not generally obligated to help them in obtaining this medical evidence. Some veterans, who could not afford a private doctor, were placed in a no win situation. They could not receive disability compensation until they submitted a medical diagnosis of their current disability; they could not get the VA to provide them with a free medical examination to obtain this diagnosis because veterans who already had service-connected disabilities were more likely to receive free VA medical care; and they could not obtain a medical diagnosis from a private doctor because they could not afford to pay for the private doctor.
As a result of the VCAA, most veterans who file an original claim for disability compensation do not need to obtain a medical diagnosis on their own. The VA is generally obligated to provide veterans with a VA medical examination to diagnose the current medical condition. There are only a few legitimate reasons for which VA may refuse to schedule a VA medical examination." Veterans Benefits Manual

1. and a medical nexus connecting 1 and 2.

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